74 



HARDWICKE'S SCIENCE-GOSSIP. 



[Apkil 1, 1869. 



the 'tegumentary skeleton of tbe Crustacea. Like 

 the bony skeleton of the vertebrata, it consists of a 

 great number of distinct pieces connected together 

 by means of portions of tbe epidermic envelope 

 which have not become hardened, in the same way 

 as among the higher animals certain bones are con- 

 nected by cartilages. In the typical crustacean 

 this external framework is divided into twenty-one 

 rings or segments, more or less easily separable or 

 distinguishable from each other : the common shrimp 

 affords us a ready and familiar example, " la squille 

 est de tous les crustaces celui oil les vingt-et-un 

 segmens du corps sont les plus distincts," says 

 Milne-Edwards. We all remember how in many 

 other instances and in other members of the animal 

 kingdom, several of these segments coalesce or 

 become fused together to form the head or the 

 carapace. These twenty-one segments, by a kind 

 of general agreement, are placed in three great 

 divisions of seveneach, and are commonly spoken of as 

 the head, the thorax, and the abdomen, by people who 

 are content to use common terms ; or as the 

 Kt(pa\ov, 7rtpalov } and ir\kov, by others who prefer 

 somewhat dubious Greek. 



The common crabs and other walking genera 

 that frequent the beach or the sea-bottom, whose 

 main strength and organs of locomotion spring from 

 the thoracic regions, have the abdomen but slightly 

 developed, and are called BracJiyura — short-tailed — 

 while the swimmer families, whose largely developed 

 abdomens are furnished with powerful muscles and 

 natatory appendages, are styled Macroura — long- 

 tailed, — the intermediate families being known as 

 Anomoura — irregular-tailed. We need not refer to 

 a host of other distinctions ; suffice it to add that 

 our " subject " wearing her visual organs in her 

 head, and not on the top of long stalks, like some of 

 her distant cousins, belongs to the Edriophthalmia 

 — sessile eyed— section ; and in virtue of possessing 

 two sorts of feet ; viz., ambulatory feet on the 

 thorax, and natatory feet on the abdomen, belongs 

 to the order Amphipoda — both-footed. 



The head of this pelagic crustacean is exceedingly 

 elongated, and its axis, instead of coinciding with 

 that of the body, is at right-angles to the latter ; 

 the back of the head is very largely developed, the 

 rounded fulness of the poll containing a mass of 

 eye-facets, the rounded tapering columns from which 

 convene and blend with the rays of the rather small 

 black lateral eyes, which are placed immediately 

 above the mouth, the aperture of which is situated 

 at the inferior extremity of the head. Two minute 

 bi-jointed antennse, with short stiff hairs or setae on 

 the terminal joint, arise from the outer margin of 

 the head, just above and in front of the lateral eyes. 



The maxillary limbs constituting the jaws, corres- 

 pond with those of ordinary masticating crustaceans, 

 and need not be noticed in detail. The seven seg- 

 ments of the thorax bear seven pairs of legs, the two 



first pairs of which are gnathopods subservient to the 

 prehension of food; the third pair are usually thrown 

 forwards across the head, for a special purpose, as 

 we shall see ; the fourth pair are the longest of all ; 

 the fifth are stoutly chelate, possessing a consider- 

 able range of motion, but appear to be normally 

 directed backwards, with the nanus downwards, 

 and the pollex, or moveable thumb, superior : the 

 sixth and seventh pairs resemble the third and 

 fourth, and terminate in simple claws ; the seventh 

 pair being commonly thrown back across the abdo- 

 men, for the purpose of antagonising the third pan- 

 before mentioned. 



The first three segments of the abdomen are each 

 furnished with a pair of natatory feet — swimmerets, 

 — the footstalks— protopodites, — of which, are very 

 large and have an inflated appearance ; the three 

 next segments taper away and are but slightly 

 developed, having their swimmerets modified into 

 mere bifid setaceous appendages; the diminutive 

 telson which terminates the abdomen being, as usual, 

 devoid of any. 



The fourth, fifth, and sixth thoracic segments 

 bear each a pair of respiratory vesicles or branchial 

 sacs of a laterally compressed oval shape, connected 

 with the posterior and inferior part of the epimeral 

 plates behind the articulations of the corresponding 

 limbs, the posterior pair are the largest, and the 

 anterior pair the smallest ; in each a loop of blood- 

 vessels may be traced, the exterior of the sac being 

 invested with a tesselated pavement of epithelial 

 cells with large nuclei. Milne-Edwards states that 

 there are five pairs " d' appendices vesiculeux," each 

 of the seven thoracic segments " excepte le premier 

 et le septieme" being furnished with them; but 

 the specimen. now before us has very decidedly only 

 three pairs, and the microscope fails to detect any 

 trace of rudiments even of others. 



The mouth, the large chela;, and the remarkably 

 stout footstalks of the swimmerets are tinted a deep 

 rich red approaching to purple, the pigment in this in- 

 stance is not amorphous, but is contained in beautiful 

 stellate cells, the remainder of the body is hyaline. 



Here we have Phronima disporting herself in a 

 glass vase of her native element : the tough 

 gelatinous transparent barrel-shaped tube open at 

 both ends which we see her carrying, is apparently 

 a portion of the tube of " the aggregate salpian " 

 Pyrosoma, from which the zooids have been 

 washed away; just as our common Hermit-crab, 

 Pagurus Bernardus, utilises a wrecked and empty 

 whelk-shell to shield his unarmoured hindquarters, 

 so does this most remarkable oceanic crustacean use 

 the castaway covering of the stolon of a Tunicary as 

 a shelter for her young : probably it is the female 

 only that we find thus accoutred with a ready-made 

 second-hand midamental case. I doubt if the male 

 ever assumes the office of nurse, but I regret to say 

 that I cannot speak positively either way. The 



