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 lamellie 

 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, l)ut 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 ocelom. 



XI. Ontogeny. 



The development of the Enteropneusta is sometimes connected with metamor- 

 phosis, a pelagic larva, the Tomaria 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 Tomaria. 



A. Structure and Metamorphosis of the Tomaria larva. 



Fig. 465.— Very young 

 specimen of Tomaria 

 Krohni, from the side 

 (after Spengel). 1, Ap- 

 ical plate with eyes ; 

 2, preoral area ; 3, pre- 

 oi-al ciliated ring ; 

 4, cesophagns ; 5, mouth ; 

 11, stomach intestine ; 

 7, anus; S, hind -gut; 

 '.I, postoral ciliated ring ; 

 10, postoral area ; 11, 

 proboscis pore ; 12, pro- 

 boscidal coslom ; 13, 

 muscle to apical plate. 



The egg segmentation and gastrula are unknown. 

 1. Outer organisation. — The youngest larval stages 

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

 pole there is a pair of brown eye-spots, at the posterior the 

 anal aperture, and in the middle of the ventral side the 

 long transverse mouth. The thin transparent integument is 

 thickened only in the region of two ciliated rings, which 

 border, in a manner soon to be described, a somewhat 

 deepened oral area, at whose centre the mouth lies. The 

 ciliated rings are strictly bilaterally symmetrical. A preoral 

 ciliated ring runs from the anterior ventral edge of the oral 

 area forwards and upwards on each side to the frontal region, 

 Avhere the eyes lie, and marks off 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 ne.xt 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 

 postoral 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). 



