590 



COMPARATIVE ANATOMY 



CHAP. 



of the proboscidal ccelom thus come into existence. The "heart vesicle" on its 

 lower side, however, always remains separated from the dorsal wall of the ventral 

 posterior outgrowth of the water sac by a space, into which the proboscidal 

 diverticulum of the buccal cavity grows from behind, but in such a way that 

 between it (the diverticulum) and the superimposed "heart vesicle" a space 

 remains, which appears to become filled with blood at an early stage. This is the 

 central blood sinus of the proboscis. 



The ccelomic sacs of the collar and trunk. These two pairs of ccelomic sacs 

 appear, in Tornaria, to have a common rudiment in the following way. The 

 edge formed by the anterior wall of the hind-gut, bending round backwards into its 

 lateral walls, is produced anteriorly to right and left as hollow sacs, or in other cases 



as a pair of solid bilaminar plates. These become 

 applied to the stomach, but are on the other 

 hand separated by a large space from the ecto- 

 derm. These two sacs or plates become con- 

 stricted from their matrix, the hind -gut, and 

 grow round the stomach dorsally and ventrally. 

 In each, apparently, an anterior portion becomes 

 constricted off. This anterior pair of sacs or 

 plates is the rudiment of the collar ccelomic 

 sacs, the posterior, which only secondarily 

 extend backwards along the sides of the hind 

 gut, is the rudiment of the trunk ccelomic sacs 

 (Fig. 467). These two cceloms are therefore 

 enteroccels. Where the first rudiments of the 

 cceloms are solid bilaminar plates, a space soon 

 arises in them by the separation of the two 

 laminae. These small spaces, whether present 

 from the first, or formed later, begin to increase 

 in size at the end of the larval period. The 

 two pairs of ccelomic rudiments become vesicular. 

 The outer wall becomes applied to the body 

 epithelium as the transverse section of the 

 growing larva decreases during metamorphosis 

 in the way already described, and forms the 

 dermomuscular tube. The inner wall lies upon 

 the intestine, and represents the visceral layer 

 of the ccelomic sac. The dorsal and ventral 

 mesenteries are formed where the right and 

 left trunk ccelomic sacs and the right and left 



collar coelomic sacs, in surrounding the intestine, come in contact dorsally and 

 ventrally in the median plane. 



These processes, of course, go hand in hand with a progressive reduction of the 

 blastoccel, which contains a number (small at first, but increasing later) of 

 mesenchyme cells of unknown origin. The remains of the segmentation cavity 

 represent the blood vascular system. 



Nervous system. Shortly before the conclusion of metamorphosis, the two 

 longitudinal nerve trunks arise as local differentations of the body epithelium, 

 below the surface of which a layer of nerve fibres forms. The collar cord, also, at 

 first lies superficially in the integument, and is nothing more than the collar 

 portion of the dorsal epithelial Ipngitudinal cord. This part only sinks below the 

 surface at a later stage. According to recent observers, the process recalls the 

 sinking in and constriction of the neural tube in Vertebrates. 



FIG. 467. Collar and trunk of an 

 Enteropneustan (Tornaria Krohni) 

 immediately after metamorphosis, from 

 the ventral side (after Spengel). 1, Pro- 

 boscis ; 2, collar ; 3, trunk ; 4, collar 

 coelom ; 5, septum between the collar 

 and the trunk ; 6, trunk coelom ; 7, ven- 

 tral mesentery ; 8, principal ciliated 

 ring ; 9, wall of the mid-gut ; 10, wall 

 of the hind-gut ; 11, anus. 



