70 HUBERT LYMAN CLARK ON 



by Seinon ('87) from the tentacles of various synaptids. These become very abundant 

 in the older ten-tentacled larvae. 



The digestive tract meantime has increased in length, and the stomach is more 

 clearly marked off from the intestine and oesophagus. The nervous system has not 

 undergone any marked changes, but each of the accessory tentacles is supplied with a 

 nerve on its inner side, as in the case of the primary tentacles. In various parts of the 

 skin, especially anteriorly, clusters of ectoderm cells are to be found which later form 

 the so-called sense-papillae (" Tastpapillen " of Hamann, '83). I have been unable to 

 find any connection between these spots and the nerves until a very much later period, 

 and I cannot decide how or when this connection is made. The glandular oi-gans 

 previously described are very abundant at this stage, especially posteriorly. Soon after 

 the pentactula form is complete, the walls of the coelom and of the hydrocoel begin the 

 formation of muscle fibers, always on the side turned from the cavity which they enclose. 

 The first to appear are the longitudinal muscles of the tentacles and radii. The former 

 appear as fibers on the outside of the tentacle canals and they soon form quite a thick 

 layer. The radial muscles arise within a fold of the coelomic wall, which appears along 

 the inner side of the radial nerves. This fold begins anteriorly near the calcareous ring 

 and runs backwards with the nerve, enclosing a considerable space between its walls. In 

 this space the muscle fibers arise from the endodermal cells of the coelom. Later on, 

 the circular muscles of the body appear, arising from the outer side of the coelomic wall 

 also. They cross the space in which the longitudinal muscles lie, forming a layer 

 between the latter and the nerve. At the same time, the longitudinal and circular 

 muscles of the digestive tract, and the muscles of the water-ring, begin to appear, so that 

 by the time the ten-tentacled stage is reached, the musculature is practically that of the 

 adult. With the appearance of the longitudinal muscles of the tentacles, comes the 

 development of the valves at the openings of the tentacular canals, close to the upper 

 edge of the calcareous ring. 



Before the accessory tentacles have begun to appear, there arises on the right hand 

 side of the mesentery which fastens the intestine to the wall of the left interradius a 

 longitudinal fold or evagination of the epithelium close to the intestine (Fig. 58). This 

 fold follows the course of the intestine, with the mesentery on the dorsal side, and grows 

 forward along the stomach and backward toward the anus. Later a similar fold appears 

 in the coelomic epithelium on the opposite (ventral) side of the digestive tract and the 

 two folds soon become connected around the intestine and stomach by numerous 

 ill-defined lacunae between the coelomic wall and that of the digestive organ itself. 

 These vessels are the first stages of the blood vascular system and into them cells from 



