ANATOMY 243 



The animal consists of a broad prostomium, with a pair of 

 eyes ; and of a body, distinctly constricted in immature speci- 

 mens into five or six segments, followed by a short conical tail. 

 There are neither chaetae nor tentacles ; locomotion is chiefly 

 effected by means of the bands of cilia which encircle the body 

 in a regular fashion, two bands round the head, and two round 

 each segment in D. taeniatus ; in some species there is only a 

 single band on each segment. The whole of the ventral surface 

 is covered with cilia, by the aid of which the animal probably 

 " creeps " along the weeds. 



The alimentary canal is straight, and divisible into the regions 

 shown in Fig. 121; a muscular protrusible organ, which is a 

 ventral outgrowth of the foregut, is employed as a "sucker." 

 The coelom is more or less obliterated (or ill developed). The 

 excretory system in the genus is varied : in some species, as in 

 D. gigas, it is stated to be constructed on the Planarian plan ; in 

 others, as in D. taeniatus, the organs are definite nephridia. Of 

 these tubes there are five pairs, the last pair in the male serving 

 as a seminal vesicle. Each nephridium is a ciliated tube, the in- 

 ternal end of which lies in the body-cavity and appears to be 

 blocked by a ciliated tongue-shaped appendage. The first pair 

 corresponds to the " larval nephridia " of Trochosphere larvae. 



The nervous system, which is in contact with the epidermis, 

 consists of a brain in the prostomium, and, on each side of the 

 body, a ventral cord with five ganglia, connected by transverse 

 commissures in as many segments. 



The sexes are separate, and are usually similar ; the male of 

 D. gyrociliatus is, however, much smaller than the female. The 

 generative organs occupy the greater part of the body-cavity ; in 

 the male the testes communicate, by means of the pair of seminal 

 vesicles, with a median eversible apparatus. In the female the 

 paired ovaries communicate with a median sac which serves as a 

 spermatheca. 



The development is simple : * the worm itself is more like a 

 larval Polychaete than a full-grown worm. Dinophilus is an 

 extremely interesting form, and it has been suggested that, while 

 still possessing certain Planarian characteristics, it may be looked 

 upon as closely resembling the ancestor from which the Chaeto- 

 poda have arisen. 



1 Schimkewitsch, Zeitschr.f, wiss. Zool. lix. 1895, p. 46. 



