THE ISOPODA. 



211 



Fig. 32. TANAIS, A REMARKABLE ISOPOD. 



A, the flabelliform appendage under the carapace in the rudimentary 

 branchial chamber. 



Although its ravages had gone on for centuries, 



depth from which Bathynomus was obtained, the eyes are greatly developed, each being made up of 

 about 4,000 square facets, and instead of being placed on the upper surface of the head, as in all 

 known wandering Cymothoidce, they are placed below the frontal border of the head at the base of 

 the antennae. Alphonse Milne-Edwards places Bathynomus in a new family of the division Cymo- 

 thoidce, named Cymothoidce branc/tiferce. 



In Tanais (Fig. 32) an aberrant form of Isopod the first pair of legs are converted into 

 chelae, the six other pairs being simple, as in other Isopods. This peculiarity, and the confluence 

 of the head with the first segments of the body, 

 give it a very Macrouran aspect. In some the 

 eyes are prominent, and almost pedunculated. 

 This group also presents many points of affinity 

 with the AMPHIPODA. 



To the Isopodous division belongs the Lim- 

 noria terebrans, or the " Gribble," as it is commonly 

 called by fishermen. It is a most destructive 

 creature, attacking all woodwork below tide-mark, 

 the only wood which it cannot destroy being teak, 

 it was only in 1811 that it was discovered and described by Dr. Leach. 



The Asellus aquaticus is a very abundant form, inhabiting fresh- water ponds and ditches. The 

 eggs and the young are retained in the pouch of the mother for about six weeks ; pi-obably half this 

 period elapses before the young quits the egg. Asellus does not exceed six lines in length, and little 

 more than half a line in breadth. It feeds upon vegetable matter exclusively. The leaves of the 

 beech in decay are preferred, and in the parts of the pond where these are most abundant, there the 

 Asellus is most numerous. The animal does not generally swim, it runs freely and expeditiously 

 over the decaying leaves. 



In Arctiirus the young are carried by the parent in rows upon the long joints of its antennae, 

 the mother remaining in a nearly erect position clinging to a branch of some zoophyte or seaweed, 

 along which she can walk by means of her hind feet. Until the discovery of Batliytwnius giganteus 

 by Agassiz (a form belonging to the Cymothoidce), the Idoteidce were supposed to contain 



representatives of the largest known Isopods, some 

 of which measure about four inches in length. 

 The hinder segments are welded together so as to 

 form a long caudal shield, beneath which are two 

 plates covering the branchiae. 



The genus Sphceroma, the members of which 

 are vegetable feeders, are also found guilty of 

 destroying timber. When molested or alarmed, 

 they roll themselves up into a ball. This genus 

 and its allies offer many points of analogy, if not 

 of affinity, with the extinct Trilobites. 



Several species among the Water Breathers are 

 parasitic, often on members of their own class. 

 Thus in Bopyrus (Fig. 33, A) the female (which is 

 six times as large as the male) is parasitic within 

 the branchial chamber of the Common Prawn, and 

 out of six Prawns, it is no uncommon thing to 

 find on? or more with this parasite distorting the carapace of the Prawn on one side. When the 

 Prawn moults his shell, the Bopyrus manages to retain its situation, and re-appears with the new 

 shell of the Prawn accommodating itself to the form of the Bopyrus. Pacjurus, Galathea, Callia- 

 nassa, Porcellana, Palcemon, and Hippolyte, all have these parasitical Crustacea in their branchial 

 chamber. Another genus (Pkryxus, Fig. 33, B) attaches itself beneath the tail of the Prawn, 

 and we have taken it also from beneath the abdomen of the Common Shore Crab at Torquay. 

 It is more curious still to note a parasitic Isopod, the female of which occupies the cavity within 



33. PARASITIC ISOPODS : BOPYRUS (A), PHRYXUS (B), 



CHYPTOTHYHIA (c). 

 aa, nmles ; W>, females ; c, larva. 



