IV 



VEEMES— ONTOGENY OF THE JFORMS 



269 



endoderm and ectoderm, i.e. deeper into the segmentation cavity. This gastrula 

 formation takes place in sucli a, way that the blastopore is a median ventral 

 longitudinal slit. This closes from back to front, until anteriorly, i.e. excentrically, 

 only a small aperture remains. An equatorial ciliated ring, the preoral ciliated 

 ring, appears early in the gastrula larva. In the cell thickening which carries this 

 ring of cilia a circular nerve is developed. At the aboral pole the ectoderm thickens 

 to form the neural plate, which carries a tuft of cilia. Directly round the narrowed 

 blastopore the ectoderm becomes invaginated like a funnel, and forms the larval 

 oesophagus or the stomodseum, which gi-adually approaches the anterior ventral 

 edge of the preoral ciliated ring. The arohenteron elongates downwards and back- 

 wards. Several smaller cells are severed by fission from the primitive or pole cells of 

 the mesoderm, and these are spread out in the segmentation cavity (primary body 

 cavity) and form various larval organs, e.g. muscle fibres and the larval head 



Fig. ns.—A, 11, C, Three stages of development of tlie larva (Trocliophora) of Eupomatus, 

 from the side, m. Polar cells of the mesoderm ; r/ul, mid-gut ; fh, segmentation cavity ; sp, neural 

 plate ; wk, preoral ciliated ring ; st, stomodajum ; wfci, postoral ciliated ring ; n, larval head nephri- 

 dium ; ot, otolith ; an, anus (after Hatscliek). 



nephridia. The primaiy body cavity lengthens. That half of the body which 

 lies behind and below the preoral ciliated ring assumes a conical form. The 

 point of the cone is the posterior end. From the posterior end to the mouth 

 the body becomes flattened. On the dorsal surface the posterior end of the 

 intestine opens by means of a small invagination of the ectoderm, the proo- 

 todasum with an anus ; this occurs not far from the posterior end of the larva. 

 Diametrically opposite to this, at the neural area, which is surrounded by the preoral 

 ciliated ring, lies the neural plate, which represents a part at any rate of the rudiment 

 of the brain. A pigment spot (eye) arises asymmetrically in a cell of the neural 

 area. Behind the mouth a slighter postoral ciliated ring appears. The whole 

 ectoderm between the preoral and postoral ciliated rings is covered with short cilia, 

 and thus forms an adoral ciliated zone. A narrow medio-ventral ciliated band forms 

 from the mouth to the posterior end. Two auditory vesicles are developed out of two 

 ectodermal cells immediately behind the postoral ciliated ring, and sink beneath the 

 surface later. Two cell streaks or bands are developed from the two pole cells of 



