THE CRUSTACEA 



a'. 



n. 



As an example, we may take Enterognathus, recently described 

 by Giesbrecht. This form differs from most other Ascidicolidae in 

 infesting, not a Tunicate, but the Crinoid Ardedon rosacea. The 

 female (Fig. 50) has an elongated body which presents the full 

 number of segments. Antennules and 

 antennae are short and consist of few 

 segments. The mouth-parts are not suc- 

 torial. The mandible consists of a long 

 and narrow blade, with toothed cutting 

 edge and a vestigial palp of two seg- 

 ments. The maxillipeds are absent. The 

 first four pairs of thoracic feet are 

 short and biramous, with the endopodite 

 forming a broad, spoon-like plate, without 

 setae, while the exopodite forms a strong 

 curved claw. By movements of these feet, 

 and by elongation and contraction of the 

 body, the parasite pushes its way through 

 the contents of the intestine of its host. 

 The last pair of feet are broad lamellae 

 fringed with hairs, covering the point of 

 attachment of the egg-masses, and prevent- 

 ing these from being detached in the 

 movements of the animal. The abdomen 



e, adnit ends i* 1 a well-marked furca, bearing a few 



female, x" 17. (After Giesbrecht.) short Setae. 



II, VI, second and sixth thoracic _,, 7 . . . 



somites; i, first abdominal Inc 1MU6 is tree-swimming, and pre- 



- sents the typical Copepod form, with fully 

 segmented body and natatory thoracic 



feet. Its most striking feature is the entire absence of mouth- 

 parts, there being no trace of the appendages between the antennae 

 and the first pair of swimming-feet. 



The life-history is as follows. The earlier stages are unknown, 

 but it appears that the first Copepodid stage is free-swimming, and 

 that both sexes enter the alimentary canal of the host in the second 

 Copepodid stage. Retrogressive changes then take place, the suc- 

 ceeding stages of the female resembling the adult in general shape, 

 while those of the male lose the natatory setae of the thoracic feet. 

 In the adult male, however, these are reacquired, while the mouth- 

 parts disappear and the animal escapes from the host. Where it 

 meets with the female is not definitely known, but it is at least 

 probable that the latter temporarily leaves the alimentary canal of 

 the Crinoid, and clinging to the surface of its body, is there fertil- 

 ised by the male. 



In certain families of the truly parasitic forms with suctorial 

 mouth-parts (Caligidae, Lernaeidae, Lernaeopodidae) a community 



