74 HUBERT LYMAN CLARK OX 



the right ventral interradius. The nerve rin^ is drawn upwai'd with the growth of tlie 

 tentacles, so that it coines to lie very near tlie ectoderm at their base. The increased 

 length of the tentacular canals has pushed the water-ring downward so that it lies some 

 distance below the calcareous ring (Figs. 90 and 91), but it is still in open communica- 

 tion with the exterior by means of the water-canal (Fig. 66). Mesenchyme cells have 

 formed a few calcareous rods about the latter (Fig. 49), especially near the point where 

 it passes into the body-wall. Just within the body-cavity from this point, openings have 

 appeared on it which place its interior in direct communication with the body-cavity, so 

 that the water-vascular system combines the primitive external opening of the pentactula 

 with the internal madreporitic openings of the other holothnrians. The mesenchyme 

 cells around the calcareous ring have formed on its posterior edge a connective-tissue 

 ring, which later becomes so prominent in the adult as the cartilaginous ring. 



In concluding this account of the embryology it may be well to summarize briefly 

 the derivation of the different organs from the germ layers of the gastrula. 



Ectoderm. From the gastrula-ectoderm arise the ectoderm of the adult, the sensory 

 epithelium of the tentacles, the entire nervous system including all the sense organs, 

 the larval glandular organs, and a small part of the oesophagus. Possibly the 

 extreme posterior part of the rectum is also ectodermal. 



Endoderm. From the gastrula-endoderm arise first of all the scattered mesenchyme cells 

 which make up the mesoderm. Soon afterwards the hydroenterocoel is divided off. 

 The remainder of the archenteron forms simply the lining of the digestive tract, 

 including most of the oesophagus. From the hydroenterocoel, the coelomic ^^ouches 

 are constricted off, leaving behind the hydrocoel, from which the entire water- 

 vascular system, and also the cavities of the digits, arise. The longitudinal muscles 

 of the tentacles and digits come from the epithelium of the hydrocoel. The coelomic 

 pouches form the peritoneal lining of the body-cavity and the epithelial covering for 

 the various organs contained in it. All the muscles of the body-wall, gut, genital 

 glands, water-ring, and Polian vessels are also derivatives of the endoderm. The 

 genital organ, including the genital duct, nnd the ciliated funnels are likewise 

 derived from the wall of the coelom. The haemal system is also covered by the 

 epithelium of the coelom and apparently arises as evaginations of the same, while 

 the blood-corpuscles certainly come from that layer. 



.Mesoderm. From the mesenchyme cells, arising from the archenteron of the gastrula, 

 come all the connective tissue of the body, the pigment, the covering of the eyes, 

 all the calcareous concretions (including the calcareous ring), and the cartilaginous 



