233 



CRUSTACEA. 



CRUSTACEA. 



234 



but is more commonly extended. This form is as natatory as the 

 first. They are frequently found congregating around floating sea- 

 weed, the buoys and strings of the crab-pot marks, and other floating 

 substance", both near the shore and in deep water. Their general 

 form somewhat resembles a Galathea." 



Subsequently to this second change a third takes place, in which 

 the animal loses its tail, and becomes more like to the form it assumes 

 in adult age. In the various species different forms are assumed, but 

 they can all be referred to departures from the typical form of the, 

 family. On this point the following observations of Mr. Couch are 

 very interesting : 



" So far as my observation has extended, it appears probable that 

 the metamorphosis of the young in their progress to adult growth is 

 uot universal in all Crustaceans ; but, on the contrary, that the families 

 in which the eyes are always sessile in their adult growth, and which 

 do not exuviate or voluntarily throw off their limbs, are in the habit 

 of producing their young perfectly formed : and an opportunity that 

 has occurred to me of observing the process of early development in 

 the common lobster will tend to establish the existence of a law of 

 nature as applicable not only to it, but probably also to all the genera 

 of this extensive family or class, that is, the Long-Tailed Crustacea ; 

 which law is, that tlie greatest extent of metamorphosis is in those 

 genera which are of the highest rank in the series, that is, the Short- 

 Tailed, or Crabs, that even at their birth the Long-Tailed genera, as 

 the Lobster, approach more closely to the ultimate size of the parent ; 

 and, what is still more extraordinary than all beside, that so long as 

 the lobster in particular retains the eyes sessile, the progress of 

 development and growth is conformed to what is the perpetual mode 

 of growth of the permanently sessile-eyed races ; and it is only when 

 the crust has become fully extended and hardened, and thus the 

 exuviation is rendered necessary, that the eyes become elevated on 

 foot-stalks, and the adult form and habit are completely established." 



With regard to the arrangement of the Crustacea, almost every 

 writer on this class of animals has embodied his own views in their 

 classification. Among the principal zoologists who have written on 

 the subject, the names of Cuvier, Desmarest, Latreille, and Leach will, 

 with many others, occur to the observer. We select the arrangement 

 of M. Milne-Edwards, because it is founded on anatomical investi- 

 gation, and on actual experiment made in a great many instances by 

 himself and M. Audouin. He makes the Cruttacea to consist of two 

 great divisions. 



1st. Those which have the mouth furnished with a certain 

 number of organs destined in an especial manner to the prehension 

 or division of the food. 



2nd. Those which have the mouth unfurnished with special pre- 

 hensile or masticatory organs, but surrounded by ambulatory extre- 

 mities, the bases of which perform the part of jaws. We shall take 

 this second division first, because it contains but one order, namely, 

 the Xyphftsura. Example, Limnlut. 



But it is to the first division that the great mans of the Crustaceans 

 belong, and these are subdivided into two great groups. 



1st The Maxillosa, or Mandibulata, which possess a mouth armed 

 with jaws, Ac. 



2nd. The Edentata, or Ifatutellata, whose mouth is prolonged in 

 the shape of a sucker. 



I. MAXIT.LOSA. 

 The Maxillota are separated into four great sections : 



1. Podophthalmia. 



These almost always possess true branchiae ; pedunculated and move- 

 able eyes; feet or extremities vergiform, partly prehensile, partly 

 ambulatory ; and a thorax covered by a carapace. 



The PodopliJhalmia contain two orders, the Derapoda and Stomapoda. 



1. The Decapoda, whose branchiae are fixed to the sides of the 

 thorax, and are inclosed in special respiratory cavities. The oral 

 apparatus is composed of six pairs of members. There are five pairs I 

 of thoracic extremities, which are generally ambulatory. The 

 Detapoda are divided into 1st, the Srachyura (Cancer, Portunus, 

 Grapsut, Podophthalmit, Thelphusa, Grcamnus, Ocypode, Pinnotheres, 

 Main, Leiirosia, Dorype, 4c.); 2nd, the Anomoura (Dromia, Ranina, 

 Pagurus, Hippa, Remipes, Brigus, &c.); 3rd, the Macroura(Astacus, 

 Scyllarus, Palamon, Palinurut, Punaus, &c.). 



2. Stomapoda, whose branchiae are external; sometimes rudimentary, 

 or none. Oral apparatus composed in general of three pairs of mem- 

 bers. Thoracic extremities prehensile, or for swimming ; generally 

 Biz or eight pairs. (Mytis, Phyllnsoma, Squilla, Tkysanopodes, Alima, 

 CyntMa, &c.). 



2. Eilriophthalmia, 



True branchiae none, but replaced by certain portions of the extre- 

 mities modified for this in their structure; eyes sessile; thoracic 

 extremities ambulatory, almost always consisting of seven pairs ; no 

 carapace. Tin; J;''lrt'>/,kthalmia contain three orders, namely, the 

 A mphipoda, the Lcemodipoda, and the Isopoda. 



1. Amjihipoda. These have the flabella of the thoracic extremities 

 vcHicular, and subserving respiration. The abdomen is very much 

 developed, subserving locomotion, and is furnished with six pairs of 



limbs, the first three of which differ in form and use from the last 

 three. (Grammarus, Talitra, Uyperia, Pltronima, &c.) 



2. Lcsmodipoda. Abdomen rudimentary. Flabella of the thoracic 

 extremities vesicular, and subserving respiration. (Proto, Caprella, 

 Cyanus, &c.) 



3. Isopoda. Abdominal extremities well developed ; the first five 

 pairs lamellar, and subserving respiration. Abdomen well developed. 

 (Idotea, Spheroma, Cymothoa, lona, Bopyrus, &c.) 



3. Branchiopoda. 



No true branchiae, but thoracic extremities lamellar, membranous, 

 and so formed as to be subservient to respiration. The Branchiopoda 

 contain two orders, Phyllopoda and Cladocera. 



1. Phyllopoda. No bivalve shell-like covering. Extremities nata- 

 tory, and in considerable numbers (from 8 to 22). (Limnadia, Cluro- 

 cephalus, Nebalia, &c.) 



2. Cladocera. Carapace in form of a bivalve-shell. Thoracic 

 members five pairs. (Daphnia, &c.) 



4. Entomostraca. 



No branchiae nor any modification of organ apparent to supply 

 the place of these. Eyes sessile, and commonly united into a single 

 mass. The Entomostraca contain two orders, namely, the Copepoda 

 and Oitrapoda. 



1. Copepoda. Body divided into distinct rings, neither carapace nor 

 valvular envelope. Thoracic and oral members in considerable num- 

 bers. (Cyclopi, Pontia, &c.) 



2. Oatrapoda. Body without very eviilent annular divisions, and 

 entirely inclosed under a large dorsal shield having the form of a 

 bivalve-shell. Extremities in very small number. (Cypris, &c.) 



II. EDENTATA. 



The Edentata contain three orders, naniely, the Araneifarmes, the 

 Sipkonostomata, and the Jjemffiforqies. 



1. Araneifarmes. Extremities rod-like, long, adapted for walking. 

 (P ycnogonon, Nymphon.) 



2. Siphonoitomata. Extremities not adapted for walking ; partly 

 lamellar, partly prehensile. (Caligua, Dicheleation, Nicothoa, &c.) 



3. Lfrnceiformet. Extremities rudimentary, body presenting auor- 

 mal forms. (Lerncea, &c.) 



Fossil Crustacea. 



Various forms of Crustacea have been found throughout the whole 

 series of fossiliferous rocks. Although their shells are not so well 

 calculated to resist decomposition as those of the Mollusca, and even 

 the Echinodermata, yet a considerable number of species have been 

 recorded, especially of the smaller forms. Bronn, in his list of extinct 

 and recent species of the families of animals, gives the following as 

 the result amongst the Crustacea : 



Extinct. Recent. 



Entomostraca 563 143 



Malncoitraca 244 541 



These numbers are probably higher for the extinct and lower for the 

 recent than the present state of our knowledge would warrant. 



One of the most interesting groups of extinct Crustacea, are those 

 found in the Silurian Rocka, and which from their most prevalent 

 forms may be called Trilobitic. [CHIROCEPHALCS ; TRILOBITES.] The 

 species in this formation are more abundant than at any subsequent 

 period, and present greater departures from the types of existing 

 Cruttacea. 



In the Devonian Rocks the Crustacea are represented also by Trilo- 

 bitic forms, some of which, as Brontes, are characteristic and remark- 

 able. This fossil which was at first supposed to be a fish, has been 

 referred by Agassiz to the Cruttacea. It was not unlike a lobster in 

 shape, but was four feet iii length. Its claws were of gigantic size. The 

 shield was sculptured with delicate markings, looking like scales. The 

 tail was continuous, and so large that a lobster of ordinary size might 

 stretch its entire length on it. 



The Carboniferous group of Rocks presents us with a considerable 

 number of species of Crustacea, but they principally belong to the 

 groups of smaller forms referred to the Entomostracous Crustacea. 

 [ENTOMOSTRACA.] We have however, in certain forms, as in those 

 species which have been referred to Apus, Asaphus, Daphnia, Cypris, 

 and Limulut, approaches to the forms which exist at the present day. 

 Of the Ostracodous Crustacea Professor M'Coy has figured, and has 

 described twenty-two species from the Carboniferous Limestone of 

 Ireland ; M. de Koninck six species in Belgium. The Ottracoda 

 described in the Carboniferous and Silurian Rocks amount to about 

 thirty seven species. 



The Permian system, embracing the Magnesian Limestone Formation 

 of England, affords the remains of no other Crustacean but those 

 belonging to the Entomostraca or Ostracoda. 



In the Oolitic Rocks the species of Crustacea are not numerous, but 

 the forms so closely resemble those more common at the present time 

 as to afford some difficulty in distinguishing them. The specimens 

 discovered in British rocks haveall been referred to the genua Astacus, 

 of which Professor Tennantgives four species A. leptomanusA. mucro- 

 natus, A. scalrosut, A. rottratus. The Lithographic Limestone of 



