229 



CRUSTACEA. 



CRUSTACEA. 



230 



lens. These small lenticular bodies exist independently, and are 

 perfectly distinct from the small corneal facets. In some cases they 

 might be mistaken (in the Idoteee, for example, where they may be 

 perceived singly, and with their distinct circular forms), and the 

 incautious observer led to conclude that the corneal facets are merely 

 these lenticular bodies so much enlarged that their hexagonal or 

 square forms result from their agglomeration in a point ; but there 

 are Crustacea, such as the Callianassce, in which these two elements 

 of the external cornea may be perfectly distinguished, the lenticular 

 body being of insignificant dimensions, and occupying the centre of 

 the corneal facet only. In general however the diameter of the lenti- 

 cular body is equal to that of the corneal facet itself, so that their 

 edges blend. Further, the lenticular bodies are most commonly 

 evolved in the substance of the cornea ; but there are cases in which, 

 under favourable circumstances, they may be detached from it. 

 Although the existence of these different modifications must not be 

 considered as being exclusive, inasmuch as there are certain Crustacea 

 which exhibit more than one of them at the same time, for instance, 

 stemmata and compound eyes, the latter only are the species of visual 

 organ encountered in the great majority of cases. Their general 

 number is two ; but these are occasionally united, so as to form a 

 single mass, and make the animal appear at first sight as if it had but 

 a single eye. This peculiarity of organisation can even be followed in 

 the Daphnice [BRAXCHIOPODA], in the embryo of which the eyes are 

 first seen isolated; with the progress of the development however 

 they are observed gradually to approach each other, and finally to 

 become united. Stemmata are always immoveable and sessile ; the 

 compound eyes with smooth cornea; however, although in the majority 

 of cases they present the same disposition, now and then occur move- 

 able : sometimes they are supported by a pedicle, moveable in like 

 manner and provided with special muscles. The eyes with facets 

 present the same modifications, and even supply important characters 

 in classifying these animals : thus in the Edriophthalmia the eyes are 

 always immoveable and sessile, whilst in the Decaf oda and Stoma- 

 poda they are supported upon moveable stems of very various 

 lengths, and which every consideration leads us to view as limbs or 

 appendages of the first cephalic ling. It sometimes even happens that 

 in these animals, between the outer edge of the carapace and the base 

 of the antennae, there occurs a furrow or cavity, within which the eyes 

 may be withdrawn or laid fiat, so as to be out of the way of injury ; 

 this groove or cavity is generally spoken of under the name of the 

 orbit." (' Cycl. of Auat. and Phys.') 



Absence of Eyes. Mr. Westwood has recently made known through 

 the Liumcan Society a form of Edriophthalmia inhabiting a deep well, 

 a species in which there is no external appearance of eyes whatever. 

 Mr. Newport has however, with his accustomed accuracy in dissection, 

 pointed out that even in this case a rudimentary visual organ exists 

 underneath the cephalic crust. 



In some of the forms (M aia, for instance) there is a fringe of hairs 

 on the inner side of the orbit, so placed as to perform the office of a 

 brush in wiping the eye when brought into contact with it. 



With regard to hearing, a cavity full of fluid, supplied with a nerve 

 fitted for the perception of impulses of sound, forms the basis of the 

 auditory system in the Crustacea. This apparatus appears to be 

 assisted by certain organs, elastic membranes, and rigid stems, for 

 instance, organised so as to vibrate under the action of sonorous 

 undulations, or to assist such vibrations. The long rigid stem formed 

 by the antennas of the second pair is said to assist in this function, 

 and, according to the highly interesting experiments of M. Savart, the 

 addition of such a rigid stem renders certain vibrations appreciable ; 

 but in some instances no such stem exists. In many of the forms 

 (Mala, for instance) there is an ossiculum auditus. 



In the Museum of the College of Surgeons (Gallery, No. 1559 A) is 

 a Hermit Crab (Payurus Mile*, Oliv.), prepared by Mr. Owen to show 

 the organ of hearing, which is composed of a simple vestibular cavity 

 situated at the under part of the basal joint of the external antennae. 

 The cavity is surrounded by a dense crustaceous substance, except at 

 the internal opening, where the auditory filament of the antennal 

 nerve penetrates it, and at the opposite side, where an elliptical open- 

 ing or fenestra is left, which is closed by the acoustic membrane : the 

 membranes of sound affect this membrane, and are transmitted to the 

 nerve, which is exposed on the left side. (Owen, ' Cat. of Physiolog.,' 

 series, vol. iii. part 1.) 



Every lobster-pot that is baited on our coasts affords evidence that 

 the Crustaceans are endued with the sense of smelling, but where the 

 organ is seated is doubtful M. de Blainville placed it in the antennae, 

 where it certainly does not reside, according to M. Milne-Edwards, 

 who further states that the opinion of M. Rosenthal, who ascribes 

 thn. function to a cavity which he discovered at the base of the first 

 pair of antennae, requires to be supported by direct experiment. 



Though the Crustacea have no true tongue, their selection of food 

 and the preference exercised by them, show that they are gifted witli 

 the sense of taste or a sense analogous to it. The seat of the faculty 

 is most probably that portion of the tegumentary membrane that 

 lines the inside of the mouth and oesophagus. 



The more or less calcareous crust with which the Crustacea are 

 covered forms a medium not calculated to convey external impressions 

 of any delicacy. " Neverthsless," says M. Milne-Edwards, " in front 



a, right external antenna of 

 Thelphuna flnrintilia ; i, left an- 

 tenuu of the same. Desmarest. 



of the head there are certain special organs which all the observations 

 I have had an opportunity of making upon the organisation of these 

 animals lead me to regard as parts more particularly destined to be 

 the seat of the sense of touch. These organs are the antennae those 

 slender filaments possessed of a great degree of flexibility, of motility, 

 and of sensibility. M. de Blainville was led to regard these organs as 

 the seat of the sense of smell ; but 

 direct and conclusive experiment 

 has satisfied us that the destruc- 

 tion of the antennae has no influence 

 whatever on the exercise of the 

 sense of smell : and we are on the 

 same grounds induced to believe 

 them destined to the exercise of 

 the sense of touch of considerable 

 delicacy, unless we would imagine 

 them as the instruments of some 

 quite peculiar sense, the existence 

 of which would be purely hypothe- 

 tical. The number and disposition 

 of these organs vary extremely. 

 Some of the Crustaceans at the 

 very bottom of the series are 

 wholly without antennae, or are 

 furnished with them in a merely 

 rudimentary state. Some species 

 have no more than a single pair ; 

 the normal number however ia 

 two pairs. In speaking of the 



tegumentary skeleton we have said to which of the rings these 

 appendages belong; we shall only say further here that they may 

 be inserted on the superior or inferior surface of the head according 

 to the respective development of the different pieces of which this 

 segment is composed. They do not differ leas widely in their form 

 and composition, and under this double point of view present modi- 

 fications analogous to those which we have specified as occurring in 

 the extremities." 



As a rule the sexual organs are separate in the Crustacea, that is, 

 they never co-exist in the same individual, and the reproduction is 

 oviparous. Milne-Edwards has however described a crab in which 

 the organs were male on one side and female on the other. Mr. 

 Spence Bate also informs us that he has in his possession a 

 specimen of Corystea in which all the characteristics are female but 

 with male organs. The celebrated hermaphrodite lobster also (' Phil. 

 Trans.,' 1730, p. 290) presented a different sexual organ on each side, 

 and both the male and female portion were complete. In the more 

 perfect Crustacea the analogy between the male and female organs is 

 so great as frequently to deceive the observer at first sight. 



In the male organs of the Common Crab the testis is grape-like, 

 the cluster consisting of four principal lobes formed of numerous 

 worm-like delicate canals convoluted into pellets. The first portion 

 is placed in the front of the thorax, and terminated in a large coiled-up 

 vessel situated on the side of the stomach ; behind, and connected 

 with it, is the deferent vessel, a convoluted canal of some size and 

 of a milky colour. It is twisted about the thorax, and at last pene- 

 trates the cell of the last pair of limbs, opening outwardly ou the 

 basilar piece, and again passes into the styliform organs, which are 

 true intromittent organs. The intercourse of the sexes only takes 

 place during the time that the female is moulting. 



" The female reproductive apparatus of the Crustacea," says Milne- 

 Edwards, "in the highest state of complication consists of an ovary, an 

 oviduct, and copulatory pouches. The ovaries in the Decapoda brachy- 

 ura resemble four cylindrical tubes placed longitudinally in the thorax, 

 and divided into two symmetrical pairs, each opening into a distinct 

 oviduct, yet communicating with one another by a transverse canal, 

 and by the intimate union of the two posterior tubes in a portion of 

 their length. The oviducts as well as the ovaries are of a whitish colour ; 

 they are short, and become united in their course to a kind of sac, the 

 neck of which extends to the exterior of the animal's body ; there is 

 one of these on each side, and they are known by the name of the copu- 

 latory pouches. It is into these reservoirs that the male pom's the 

 spermatozoa, which are simple round cells, and are applied to tha 

 ova as they pass in succession along and out of the oviducts. These, 

 after a course which is never long, terminate at the vulvae openings 

 formed in the sternal pieces of the segment which supports the third 

 pair of ambulatory extremities. 



" The A nomoura and Macroura have no copulatory pouches, and 

 their vulvas are situated on the basilar joint of the ambulatojy 

 extremities of the third pair. The mode in which fecundation is 

 accomplished in these genera is consequently much less apparent than 

 in the Brachyura. Many writers are of opinion that this operation 

 takes place in the interior of the ovaries, a process that appears by no 

 means feasible on account of the inequality of development of the 

 ova, which is such that the last of them are not in being even long 

 after the first have been expelled. 



" The female Crustacean does not abandon her eggs after their 

 extrusion. Those of the Decapods preserve them under their abdo- 

 men by means of the abdominal extremities modified in their 



