306 



CRUSTACEA. 



female larva the head decreases in size, the eyes undergo degeneration, while in 

 the male larva the head grows out into a large square region of the body, which 

 is also provided with degenerate eyes, and from which project, anteriorly at the 

 sides of the rudimentary upper lip, two strong seizing pincers. We should feel 

 inclined to derive these latter from the mandibles of the young form, had not 



Dohrn observed that they have an 

 independent origin (c/. the treatises 

 of Spence Bate, No. 161 ; Hesse, 

 No. 168, and Domix, No. 164). 



In those much specialised parasites, 

 the Bopyridae, which have sucking 

 and very much reduced mouth-pai'ts, 

 striking sexual dimorphism develops 

 in a manner similar to that found 

 in many parasitic Copepoda (Lernaeo- 

 podidae), the less degenerate but 

 smaller dwarf males (Fig. 147 A) 

 appearing attached to the large 

 and much deformed females. The 

 males retain in general the Isopod 

 appearance, the body remains 

 symmetrical and distinctly seg- 

 mented, the eyes are retained, 

 although in a reduced condition. 

 In the female, on the contrary, the 

 eyes are almost entirely lost ; the 

 disc-like broadened body is asym- 

 metrical, and its segments often indistinctly marked off one from the other. 

 The segmentation of the abdomen may in both sexes be reduced. 



The larvae of the Bopyridae, when they leave the brood-cavity 

 (Fig. 146), have well-developed jointed antennae, the second pair 

 being principally used for locomotion. The mouth-parts already 

 show the structure characteristic of the adult. The six pairs of 

 thoracic limbs are developed as anchoring organs. The last thoracic 

 segment is still, as in all Isopod larvae, without limb-rudiments. 

 Most of the abdominal segments are distinct from one another, only 

 the last two are fused with the telson. There are five biramose (in 

 many forms uniramose) pairs of pleopoda used as swimming limbs, 

 while the limbs of the sixth segment (uropoda) have developed as 

 caudal hooks. There are no distinctions of sex at this stage between 

 the larvae, which swim about freely and seek for their future host. 



After attachment has taken place in the branchial cavity of the 

 host (commonly a prawn), the larva develops further through 

 the appearance of the last pair of thoracic limbs, and the gnat 

 reduction of the antennae and the pleopoda, till, finally, the reduced 

 adult form above described is reached. The abdomen of the adult, 



Fig. 146.— Larva of Bnpyrus with six pairs of 

 thoracic limbs (after Walz). a', first antenna ; 

 a", second antenna; md, mandible ; ul, lower 

 lip; abs, first abdominal segment. 



