258 



ZOOLOGY 



SECT. 



be contrasted with the enibolic gastrulation of Sycon (p. 114). In 

 many cases the embryo develops into a characteristic larval form 

 known as Mutter's larva (Fig. 206). It assumes an oval shape, with 

 a series of eight elongated processes, covered with long cilia, and 

 connected together by a ciliated band. There are eye-spots at 

 the anterior end, and a mouth in the middle of the ventral 



FIG. 20*3. M tiller's larva. A, longitudinal section; B, lateral view. cc. ectoderm; t'/i.. 

 endoderm ; g. brain ; hd. enteron ; o. mouth ; ph. pharynx ; pt. pharyngeal pouch ; sn. suckeiv 

 (From Lang). 



' 



surface. The form of the body alters after a time, becoming 



Gradually longer and flatter, and the arms become gradually re- 

 uced in length, till, eventually, they become completely absorbed. 

 Little is known of the development of the Rhabdocceles or of 

 the Monogenetic Trematodes : in both groups development is 

 direct, and in many the young animal, when it escapes from the 

 interior of the egg, has already assumed the form of the parent. 

 Gyrodactylus (Fig. 189, A) is peculiar in being viviparous ; before 

 the embryo is born a second has become developed in its interior 

 and a third in the interior of that again. Even more remarkable 

 is the case of Diplozoon, in which two young animals become fused 

 together to form a double mature individual. 



An account has already been given (p. 230, Fig. 179) of the 

 development and metamorphosis of the Liver-Fluke (Distomum- 

 hepaticwm), which may b^4ootoi-aipjQn as typical of thejjigenetic 

 Trematodes in general. 



As in the majority of the Platyhelminthes, the egg-shell of the 

 Digenetic Trematodes, formed from the secretion of the shell- 

 glands, encloses not only the ovum derived from the ovary, but 

 also a number of small yolk-cells contributed by the yolk-glands ; 

 these soon lose their ceil character. The ovum has been fertilised 

 by a sperm before the shell is formed, and undergoes division into 

 a number of cells which gradually displace and absorb the yolk, 

 the latter taking no direct part in the development of the embryo,, 

 but serving only for its nourishment in the earliest stages. The 

 embryo escapes from the egg as a ciliated larva, which develops in 

 the interior of the second or intermediate host into a sporocyst. 

 This may multiply by budding or fission. The central cells of the 

 larva, which are regarded as ova that develop parthenogenetically,. 



