586 COMPARATIVE ANATOMY CHAP. 



Between these two layers lies (3) a limiting membrane, in which a rich capillary 

 network may be developed, or else the membrane is divided into its two lamella 

 by a continuous slit-like blood sinus. 



The origin of the gonads is not yet certainly known. They were formerly held to 

 be derived from the ectoderm, but the most recent researches seem to show that 

 they arise as local accumulations of the mesenchyme cells which occupy the 

 blastoccel. In any case the connection of the gonads with the body epithelium 

 by means of the ducts is secondary. They originally lie isolated between this 

 epithelium and the parietal layer of the ccelom. 



XI. Ontogeny. 



The development of the Enteropiieusta is sometimes connected with metamor- 

 phosis, a pelagic larva, the Tornaria larva, being developed. This larva in many 

 respects recalls the Bipinnaria larva of the Asteroids, and 

 was at first considered to be an Echinoderm larva. In other 

 cases development is abbreviated, and is indeed almost direct, 

 for though a free larva develops from the fertilised egg, it 

 lives at the bottom of the sea, and shows no signs of many of 

 the most important characters of Tornaria. 



A. Structure and Metamorphosis of the Tornaria larva. 



t The egg segmentation and gastrula are unknown. 



Fm. -k;.-,. Very young 1. Outer organisation. The youngest larval stages 

 specimen of Tornaria observed are almost egg-shaped (Fig. 465). At the anterior 



^fto^pengel).* 11 !, A|! l wle there is a P air of brown eye-spots, at the posterior the 

 ical plate with t'yi-s ; ana l aperture, and in the middle of the ventral side the 

 2. in-eoral area; 3, pre- long transverse mouth. The thin transparent integument is 

 oral ciliated ring; thickened only in the region of two ciliated rings, which 

 4, oesophagus; 5 month; bord in & maimer soQn b described somew hat 



t>, stomach intestine ; 



7, anus; s, hind-nut; dee l )enec l oral area, at whose centre the mouth lies. The 

 0, postoral ciliated rin^c ; ciliated rings are strictly bilaterally symmetrical. A preoral 

 10, postoral area ; ii, ciliated ring runs from the anterior ventral edge of the oral 

 proboscis pore; 12, pro- area fonvards and llpwa rds on each side to the frontal region, 

 boscidal coelom ; 13, , , .. . . 



muscle to apical plate. wnere the e y es l le > and marks oft a preoral area. A second 

 ciliated ring runs back on each side, almost longitudinally, 



from the frontal region, then bends round on to the ventral side, and here, 

 behind the mouth, passes into the corresponding ciliated band of the other side 

 of the body. 



This postoral ciliated ring forms the dorsal and posterior boundary to the oral 

 area, and marks off a postoral area, which comprises the dorsal and posterior (anal) 

 regions of the larval body. The preoral and postoral ciliated rings unite for a very 

 short distance at the apical pole. The oral area enclosed within these two rings has 

 the form of a transverse ventral saddle, drawn out on each side towards the apex. 



The next remarkable change which is externally visible is the appearance of a 

 ciliated ring at right angles to the principal axis. This surrounds the posterior 

 part of the postoral area, and is the principal ciliated ring (Fig. 466, 9). The 

 po.storal area is by it divided into an anterior and a posterior region. The posterior 

 region is the anal area, with the anus at its centre. In the anterior region, a 

 dorsal area can be distinguished from a ventral zone. Behind the principal ciliated 

 ring, a second weaker ciliated anal ring may appear (Fig. 466, 8). 



