A typical larva of the first group has the following structure 

 (Fig. 116). Its body is elongated and cigar-shaped, and it is provided with 

 three zones of ciliary epithelium. There are well-developed head glands 

 which open outside by two groups of ducts on the anterior edge of the head 

 end. The anterior end of the body has'two pairs of well-developed pigmented 

 little eyes with light refracting lenses. The buccal aperture is located 

 ventrally at the level of the first pair of eyes or even in front of it. The 

 pharynx is powerfully developed, the intestine is circular. There is a 

 weakly developed nervous system and excretory system. The sex system 

 is not developed but for the most part a group of large cells representing p. 99 



Fig. 116. Free -swimming larva 

 of the first type. Explanation in 

 text. 



Fig. 117. Free-swimming larva of 

 the first type. Explanation in text. 



the sex embryo (gonad anlage, nobis ) lies inward from the intestinal ring. 

 The attaching structure of the posterior end of the body consists of 14 to 16 

 edge hooks. They have a well-developed terminal little hook and a more or 

 less well-developed handle. The latter is firm, hard and not flexible. Its 

 growth (if it takes place in the postembryonic period) takes place at the 

 expense of a super growth (accretion, nobis) at the free end (skeleton-forming 

 cells lie near the upper end of the handle. ) 



100 



