244 ENTOPROCTA. 



oral disc and the oesophagus are richly ciliated. The oesophagus 

 first, and afterwards the rectum unite with the archenteron (fig. 130), 

 the walls of which soon become differentiated into a stomach and 

 intestine, and on the upper wall of the former the hepatic cells 

 become especially conspicuous (fig. 130). 



During the completion of the alimentary canal a number of 

 important structures is formed. The disc in which the oral and 

 anal apertures are situated becomes converted into a true vesti- 

 bule. On its floor, between the mouth and the anus, there arises 

 a marked prominence with a tuft of cilia (fig. 130 B), which per- 

 sists in the adult. 



This prominence is perhaps equivalent to the epistome of the 

 Phylactola3mata and the disc-like organ of Rhabdopleura, which 

 Lankester has compared to the molluscan foot 1 . 



Very shortly after the first formation of the vestibule there 

 appears at the opposite end of the larva a thickening of the epiblast, 

 which soon becomes invaginated, and forms an eversible pit (fig. 



129 A and B, /.</.) Round its mouth there is formed a ring of stiff 

 cilia (fig. 130, f.g.}. This organ is very possibly equivalent to the 

 cement gland described by Kowalevsky in the adult Loxosoma. I 

 shall speak of it as the ciliated disc. 



The epiblast cells early secrete a cuticle. 



The two mesoblast cells soon increase by division, and occupy 

 the space between the alimentary canal and the body wall. They 

 do not become divided into a splanchnic and somatic layer ; but give 

 rise to the interstitial connective tissue and muscles. From the 

 mesoblast there is also formed, according to Hatschek, a pair of 

 ciliated excretory canals, in the space between the mouth and anus 

 (fig. 330 B, nph.). The development of the nervous system has not 

 been observed. 



At a comparatively late stage in the development there is 

 formed round the edge of the vestibule a ring of long cilia (fig. 



130 B, m.). 



A remarkable organ makes its appearance on the dorsal side of 

 the oesophagus (the side opposite the adult ganglion) formed of an 

 oval mass of cells attached to the epiblast at the apex of a small 

 ciliated papilla (fig. 130 A and B, a?.). This organ will be spoken 

 of as the dorsal organ. According to Hatschek it develops as 

 a solid outgrowth of the hypoblastic walls of the mesenteron shortly 

 before the mesenteron joins the oesophagus (fig. 129 B, #.). The 

 cells composing it arrange themselves as a sack, which acquires an 

 external opening on the dorsal surface (fig. 130 A, x.). In a later 

 stage the lumen of the sack disappears, but at the junction of the 

 organ with the epiblast a pit is formed, lined with ciliated cells, 

 which is capable of being protruded as a papilla. The organ itself 



1 Lankester. "Remarks on the Affinities of Rhabdoplenra." Quart. J. of Micro. 

 Science, Vol. xiv. 1874. 



