IV 



VERMES— ONTOGENY OF THE WORMS 



271 



Bryozoa.— It is very difficult to describe the different, and for tlie most part still 

 insxiffioiently known, larval forms of the Bryozoa, and to establish their relations to 

 the adult animals. "We must confine our- 

 selves to a description of the larva of 

 PcdkeUina (Endoprocta) (Fig. 180). A 

 ciliated ring on a circular elevation divides 

 the larval body into two regions. In 

 the oral region, which is ciliated all 

 over, lie the mouth and anus, the latter 

 on a conical prominence. Behind the 

 mouth lies a prominence with a tuft of 

 cilia. Between the mouth and anus there 

 is a depression, the vestibular pit. The 

 whole oral region can be withdrawn into 

 the aboral, so that the month and anus 

 come to lie at the base of a depression 

 whose edge is formed by the ciliated ring, 

 and which is called the vestibulum. In 

 the middle of the aboral region rises a 

 ciliated tuft. From the base of this ciliated 

 tuft an organ consisting of long ectoder- 

 mal cells, the so-caUed cement gland, pro- 

 jects into the interior of the larva. There 

 is also in the aboral region another organ 

 projecting inwards like a sac, into which 

 a short canal enters from outside, this is 

 the so-called dorsal organ. Both these 



organs are said to disappear in later metamorphosis. In examining the inner organis- 

 ation we find a stomodseum, a sac-like mid-gut, and a hind-gut rising up to the anus. 

 The wall of the intestme turned towards the vestibulum is much thickened and is 

 called the liver. Muscles arranged in various ways serve for retracting the oral region. 

 Between the stomach-intestine and the body epithelium of the oral region lies a mass 

 of mesoderm cells, and connected with this on each side a small ciliated canal (nephri- 

 dia of the adult animal ?). If we wish to compare these Endoproctan larvEe, which show 

 considerable resemblance with the Ectoproctan larva of Membranipora known as Cypho- 



FiG. ISO.— Larva of PecUcellina (after Hats- 

 cliek), from the side, o, Mouth ; n, nephridium ; 

 lojc, ciliated ring ; do, dorsal organ ; kd," cement 

 gland" ; vjs, ciliated tuft ; I, "liver"; m, meso- 

 derm cells ; pdf proctodEeum ; an, anus ; vs, 

 vestibular pit ; st, stomodjeum ; md, mid-gut. 



Flo. ISl.— ^, B, C, D, Four stages of the metamorphosis of the attached larva of Pedicel- 

 Una, do, Dorsal organ; kd, "cement gland"; st, stomodEeum ; md, mid-gut; %id, proctodeum; 

 V, vestibulum ; vs, vestibular pit ; i, rudiment of tentacles ; pe, peduncle. The arrows indicate 

 the direction from inouth to anus, ve. Invagination of the body wall towards the vestibulum 

 (after Barrels). 



iiOMtes, with a Polychcetan TroAopJiora, we must consider the ciliated ring as equiva- 

 lent to the preoral ciliated ring of the latter. Then the so-called cement gland 



