338 



ZOOLOGY 



tubes (n'). The inner ends of these do not open into the body-cavity, but are 

 provided with peculiarly modified flagellate cells known as solenocyte*, so that 

 these paired excretory tubes resemble closely the nephridia of some of the Poly- 

 chseta (phylum Annidata ; see Section X.). The sexes are separate. In the male 

 there is a conical ventral penis ; the last pair of nephridia act as vesiculas seminales. 

 In the ovary two sets of ova are developed, the larger destined to give rise to 

 females, and the smaller destined to form males. They pass into the body-cavity 

 and reach the exterior by an aperture on the ventral surface in front of the anus. 



A process of unequal segmentation is 

 followed by the formation of an 

 epibolic gastrula. What is known 

 of the development is in favour of 

 the view that Dinophilus is to be 

 looked upon as a trochophore-like form 

 that has made some progress in the 

 evolution of metamerism. 



The Histriobdellea comprise only 

 the two nearly-allied genera Histriob- 

 della and Stratiodrilus (Fig. 275) — the 

 former found on the eggs of the Euro- 

 pean lobster, the latter in the gill- 

 cavities of Australian and Tasmanian 

 fresh-water crayfishes. The animal is 

 narrow, almost cylindrical, with a 

 well-marked head, a body of six seg- 

 ments, and a narrower tail-region in 

 which segmentation is not clearly 

 marked. The head bears five tentacles 

 {t 1 , t 2 , t 3 ) tipped with non-motile sen- 

 sory cilia, and a pair of retractile 

 appendages or limbs (I, a), with basal 

 glands the ducts of which open at 

 their extremities. The head has the 

 mouth at its anterior extremity on 

 the ventral aspect. The . body bears, 

 in Stratiodrilus, three pairs of two- 

 jointed non-retractile appendages or 

 cirri (c 1 , c 2 , c 3 ) tipped with non- 

 motile cilia, and in the male a pair 

 of retractile appendages or claspers 

 (d). At the end of the tail is a pair 

 of large freely movable appendages 

 or legs (Ip), which are the organs of 

 locomotion : at the end of each of 

 these open the ducts of a mass of 

 unicellular glands. The anus is situ- 

 ated posteriorly between the bases 

 of the legs. Opening from the mouth - 

 cavity on its ventral aspect is a 

 muscular sac in which are enclosed, 

 when retracted, a system of chitinous 

 jaws reducible to the same general 

 type as the mastax of the Rotifera, 

 but with the relative position of malleus and incus inverted. There is a 

 highly developed nervous system consisting of a large brain (br-. c.) situated 

 dorsally in the prostomium, a pair of oesophageal connectives, and a ventral 

 nerve cord (nc) with a series of ganglia which have a distinctly metameric 



Fig. 275.— Stratiodrilus tasmanicus, 



male. ac. accessory gland of male ap- 

 paratus ; br. c. brain ; gl, c 2 . c3. cirri ; cl. 

 claspers (appendages peculiar to the male) ; 

 ex. excretory tubes ; gr. gld. granule-gland ; 

 I. a. anterior limb ; I. gl. gland at base of 

 anterior limb ; I. gld. gland at base of pos- 

 terior limb ; I. p. posterior limb ; n. c. 

 nerve-cord ; p. penis ; &. i 2 . t :i . tentacles ; 

 vea. vesicula setninalis. 



