310 



ZOOLOGY 



SECT. 



to all intents and purposes, a sexually mature trochosphere with a 

 mastax. The excretory organs recall those of the Platyhelminthes, 

 and also resemble the provisional nephridia or head -kidneys of 

 Annulate larvse. Lastly, the hollow muscular appendages of Peda- 

 lion and Hexarthra give those genera a certain resemblance, 

 which is probably, however, merely adaptive, to the Nauplius or 

 free-swimming larva of Crustacea. 



Class II. DINOPHILEA. 



The various species of the genus Dinophilus are to be looked upon, like the 

 Rotifera, as modified Trochospheres. 



Dinophilus (Fig. 252) is a minute worm-like animal with a head or pro- 

 stomium, a body composed of five to eight segments separated from one another 



by constrictions, and a short ventral tail. 

 The prostomium bears two eye-spots (a) 

 and some sensory hairs ; it is either 

 covered uniformly with cilia or bears two 

 or three annular ciliated bands, apparently 

 representing the prototroch of the Tro- 

 chosphere. The body is in some of the 

 species uniformly ciliated : in others the 

 cilia are disposed in rings (>ck) correspond- 

 ing to the segments, except on the ventral 

 surface, where the ciliation is always uni- 

 form. The mouth (m), which is situated 

 on the ventral aspect of the prostomium, 

 leads into an alimentary canal consisting 

 of pharynx, oesophagus, stomach, and in- 

 testine, all of which are ciliated ; the anus 

 (an) is placed dorsally over the tail. A 

 protrusible muscular proboscis lies when 

 retracted in a recess opening close to the 

 mouth. There is a crelome which is crossed 

 by strands of connective tissue. A nervous 

 system is present, and consists of a large 

 ganglion in the prostomium, giving off two 

 anterior and two posterior nerves, or two 

 lateral cords (sometimes segmented into 

 a series of ganglia) all situated in the 

 epidermis. 



In one species (D. gigax) there is an 

 excretory system which is comparable with 

 that of the flat-worms, and contains flame- 

 cells, the internal openings of which are 

 provided with triangular ciliated appen- 

 dages ; in others there are five pairs of 

 tubular nephridia (H). The sexes are 

 separate. In the male there is a conical 

 penis ; the last pair of nephridia act as 

 vesiculne seminales. In the ovary two 

 sets of ova are developed, larger ones 

 destined to give rise to females, and 



smaller destined to form males. They pass into the ccelome 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. 



ett~~ 



FIG. 252. Dinophilns gyrociliatus, 



female, itii.aims; '.eye; a', hind-gut ; 

 m. mouth ; "'. *tom:u-li ; . nephridia : 

 11. o vary; /'/<. pharynx ; />/?. pharyngeal 

 glands ; n-k: ciliated rings. (From Lung, 

 sifter E. Meyer.) 



