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PHYLUM TROCHELMINTHES 



331 



br. 



are provided with peculiarly modified flagellate cells known as solenocyles, so 

 that these paired excretory tubes resemble closely the nephridia of some of 

 the Polychseta (phylum Annulata ; see Section X). The sexes are separate. 

 In the male there is a conical ventral penis ; the last pair of nephridia act as 

 vesiculse 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 Histrio- 

 bddla and Stratiodrilus (Fig. 280)- 

 the former found in the gill-cavities 

 and 011 the eggs of the European 

 lobster and the Norway lobster, the 

 latter in the gill-cavities of Australian 

 and Tasmanian fresh- water cray- 

 fishes. The animal is narrow, almost 

 cylindrical, with a well-marked head, 

 a body of six segments, and a nar 

 rower tail-region in which segmen- 

 tation is not clearly marked. The 

 head bears five tentacles (t 1 ., t 2 ., t 3 .) 

 tipped with non-motile sensory cilia, 

 and a pair of appendages or limbs 

 (l.a.) (retractile in Stratiodrilus), with 

 basal glands the ducts of which open 

 at their extremities. The head has 

 the mouth at or near its anterior 

 extremity on the ventral aspect. 

 The body bears, in Stratiodrilus, 

 three pairs of two-jointed non-re- 

 tractile appendages or cirri (c l .,c 2 .,c 3 .) 

 tipped with non-motile cilia, and in 

 the male a pair of retractile append- 

 ages or claspers (d.). At the end of 

 the tail is a pair of large freely 

 movable appendages or legs (l.p.), 

 which are the chief organs of loco- 

 motion : at the end of each of these 

 open the ducts of a mass of unicel- 

 lular glands. The anus is situated 

 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 cesophageal connectives, and a ventral nerve cord (n.c.) with a series of 

 ganglia which have a distinctly metameric arrangement. The excretory 



FIG. 280. Stratiodrilus tasmanicus, 



male. ac. accessory gland of male ap- 

 paratus ; br. c. brain ; c 1 . c'-. c' ! . cirri ; cl. 

 claspers (appendages peculiar to the male) ; 

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

 I. a. anterior limb ; /. <//. glanrt at base of 

 r anterior limb ; 1. gld. gland at base of pos- 

 terior limb ; 1. p. posterior limb ; n. c. 

 nerve-cord ; p. penis ; t 1 . t 2 . t 3 . tentacles ; 

 ves. vesicula seminalis. 



