68 HT'BERT LYMAN CLARK OX 



may be briefly sumiiied up as follows: — Water- vascular system consisting of a closed 

 hydrocoel ring or circumoral water-tube with five primary tentacles, between which are 

 five very much smaller but equally erect secondary outgrowths; a water-tube in the mid- 

 dorsal interradius connecting the circumoral rino; with the exterior; and a Polian vessel 

 in the left dorsal interradius. Nervous system consisting of a circumoral ring ; five ten- 

 tacle nerves on the inner face of the primary tentacles, the ectoderm of which is consid- 

 erably thickened, especially on the outer side; five radial nerves bending backward over 

 the secondary outgrowths of the water-ring and over the radial pieces of the calcareous 

 ring, and running to the posterior end of the body; and five pairs of otocysts, lying 

 external to the radial nerves, where they bend backward. Digestive system, consisting 

 of a short oesophagus with the mouth opening anteriorly in the center of the circle of 

 tentacles, a large stomach, a comparatively short intestine with a single loop in it, and 

 usually an anus formed secondarily near or at the aboral pole. Digestive system attached 

 to the wall throughout its whole course by a mesentery, formed by the union of the two 

 walls of the right and left coelomic pouches. Calcareous ring consisting of five radial 

 and five interradial pieces with much-branched ends. A few scattered glandular organs 

 of doubtful function in various parts of the ectoderm. 



7. The Development of the Adult Synapta. 



Since there is no cessation of growth nor any resting period on the assumption of 

 the pentactula form, it is impossible to draw any hard and fast lines which will always 

 serve to distinguish that stage. For many larvae, which appear to have only five tenta- 

 cles, show on careful examination the rudiments of new ones, and other larvae which 

 show no tentacles externally show the perfect pentactula form, when sectioned. As soon 

 as the five primai-y tentacles have pushed out so far as to entirely obliterate the original 

 opening of the atrium, the secondary outgrowths of the water-ring, which have hitherto 

 scarcely shown any indication of growth, begin to develop and push upward. As we have 

 already seen, the radial nerves lie directly over them, so that they cannot grow straight 

 up but push out to one side or the other of the nerve. The outgrowth which lies in the 

 mid-ventral radius, however, develops very slightly and does not -normally push out on 

 either side of the nerve which overlies it. The outgrowths which lie in the right and 

 left ventral radii take the opposite course, and broadening out laterally, grow up on both 

 sides of the nerves which overlie them, to form accessory tentacles. The outgrowth of 

 the right dorsal radius pushes out on the dorsal side of its overlying nerve and forms an 

 accessory tentacle in the mid-dorsal interradius, while the outgrowth of the left dorsal 



