590 



COMPA RA TIVE A NA TOM Y 



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

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

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

 diverticulum of the buccal cavity grows from behind, but in such a M'ay that 

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

 remains, which a]>])ears to become tilled with blood at an early stage. This is the 

 central blood sinus of the proboscis. 



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

 ajjpear, 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 jn'oduced anteriorly to right and left as hollow sacs, or in other cases 



as a pair of solid bilaminar plates. These become 

 apjilied to the stomach, but are on the other 

 liand 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, apjiarently, an anterior portion becomes 

 constricted otf. This anterior pair of sacs or 

 plates is the rudiment of the collar coelomic 

 sacs, the posterior, which only secondarily 

 extend backwards along the sides of the hind 

 gut, is the rudiment of the trunk coelomic sacs 

 (Fig. 467). These two coeloms are therefore 

 enteroccfils. "Where the first rudiments of the 

 cceloms are solid bilaminar plates, a space soon 

 arises in them by the separation of the two 

 lamina?. These small spaces, whether present 

 I'roni 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 



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

 tral mesentery ; 8, principal ciliated 

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

 of the hind-gut ; 11, anus. 



Fig. 4G7.— Collar and trunk of an 

 Enteropneustan (Tornaria Krohnl) 

 iiuniediately after nietaniurpliosis, from 

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

 boscis ; 2, collar ; 3, trunk ; 4, collar in the way already described, and forms the 

 ccBloni; .% septum between the collar dermomuscular tube. The inner wall lies upon 



the intestine, and represents the visceral layer 

 of the coelomic sac. Tlie dorsal and ventral 

 mesenteries are formed where the right and 

 left trunk ccelomic sacs and the right and left 

 collar ccelomic 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. — Sliortly before the conclusion of metamorphosis, the two 

 longitudinal nerve trunks arise as local difl'erentations 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 longitudinal 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. 



