ISOPODA. 



459 



are the first formed. After these have made their appearance, a new 

 cuticle, the larval skin corresponding to the Nauplius stage, is formed 

 (as also is the case in Ligia according to Fr. Miiller). While the 

 other appendages are successively developed, the caudal region of the 

 embryo becomes bent towards the dorsal surface. Of the embryonic 

 membranes the chorion is the first to disappear, then the cuticle of 

 the blastoderm, and finally, when the embryo is fully developed, the 

 ISTauplius skin. 



The young animals, when they become free in the brood-chamber 

 (fig. 361), are still without the last pair of thoracic legs; in the 

 Tanaidce the abdominal feet are also wanting. They undergo not 



inconsiderable changes in the form of the appendages until the 



attainment of sexual maturity. 

 The Isopoda may therefore be 



said to undergo a metamorphosis 



which is most complete in Ta- 

 nais, Praniza (Anceus) and the 



Bopyridcs. 



The Isopoda live some in the 



sea, some in fresh waters, and 



some on land (Oniscidce). They 



nourish themselves on animal 



matters ; many of them are para- 

 sitic (seldom complete enclopara- 



sites, Entoniscus) principally on 



the skin and in the buccal and 



branchial cavities of fishes (Cy- 



mothoidce) or in the branchial 



cavity of prawns (Bopyridce). 



Tribe 1. Anisopoda.* 



Body more or less resembling that of an Ampldpod. The abdomen 

 with biramous swimming feet (Tanais), which do not function as gills, 

 or with fin-like feet (Anceus). 



Fam. Tanaidae. Tanais dulius Kr., Brazil. Two kinds of males, "smellers 

 and claspers." T. gracilis Kr., Spitzbergen. 



Fam. Pranizidae, Anceidas. Anceus maxillaris Mont. (Pr. cccrulcata 

 Dcsm.), North and West coasts of Europe. 



* Compare Spence Bate, "On Praniza and Anceus, etc," Ann. of Nat. Hist., 

 Ser. 3, Vol. II., 1858. Hesse, " M&noire sur les Pranizes et les Ancees." 

 Ann. d. Scien. Nat., Scr. IV., Tom IX., 1864. Fr. Miiller, " Ueber den Bau der 

 Scheerenasseln," Areliiv. fiir Natnrgcsck, Tom XXX., 1864. A. Dohrn, 

 " Entwickelung und Organisation von Praniza maxillaris sowie zur Krfintniss 

 des Baues von Paranthura costana " Zcitsclir. fiir vriss. ZooL, Tom. XX., 1870. 



FIG. 381. Larva of Bopyrus virbii, with six 

 pairs of thoracic le#s (after R Walz). 

 Ul, Under lip ; Abs, first abdominal seg- 

 ment ; A', A", two pairs of antenna?; Mdb. 

 mandible. 



