xvi ECHINODEKMATA 539 



only partly corresponds to this ; it partly represents the space, filled 

 with gelatinous connective tissue, which separates right and left 

 horns of the left posterior coelom. The ciliated ring, which is 

 invaginated, seems to give rise to the nerve -ring and to the 

 ectodermal covering of the first five atrial tentacles, which, as they 

 increase in length, protrude into the cavity. 



According to Lyman Clark (1898), in Synapta vivipara the mouth 

 comes to protrude from the floor of the atrium as a papilla. This 

 papilla grows upwards, and its margin fuses with the sides of the 

 atrial cavity high up ; and in this way a portion of the atrial cavity 

 becomes completely enclosed in the form of a ring surrounding the 

 mouth. This ring is the epineural canal, which overlies the ring 

 nerve. There is no doubt that a similar process takes place in 

 Synapta digitata. At the same time the rudiments of the radial 

 canals (r.c, Fig. 400) grow out and bend backwards, and extend along 

 the radii. In Synapta vivipara, according to Clark, the rudiments 

 of the radial canals remain entirely vestigial, and never grow back 

 along the radii. 



The pupa swims about for a considerable time, and is only very 

 gradually transformed into the adult. As the tentacles grow in 

 length the atrial opening is widened, and they protrude freely to 

 the exterior and develop their lateral branches. At this stage the 

 pupa, as figured by Metschnikoff (1869), seems to have only four 

 transverse hoops of cilia, so that one of the five must have dis- 

 appeared. The intestine, which until now has projected straight 

 backwards, develops a knee-like bend where it issues from the 

 stomach, and the loop formed by this hinge grows forward till it 

 nearly reaches the level of the hydrocoele. In this way the ascend- 

 ing and descending limbs of the intestine are formed. 



The peculiar otocysts (ot, Fig. 401) which distinguish Synapta 

 are developed as evaginations of the epineural canal. The ring of 

 calcareous ossicles surrounding the oesophagus is developed from 

 mesenchyme cells which come to lie between the oesophagus and 

 the nerve-ring. By these cells first five radial pieces, and much 

 later five interradial pieces, are formed. An accumulation of 

 mesenchyme cells also clusters round the anterior coelom, and 

 here forms the calcifications which are known as the secondary 

 or internal madreporite. Thus the remnant of the anterior 

 body-cavity is enclosed, and it becomes pierced by pores which 

 communicate with the coelom and which are lined with ciliated 

 epithelium. The primary pore-canal and the pore disappear. 



Soon after the atrium has opened out and the tentacles have 

 become protruded, the transverse ciliated bands disappear, and the 

 pupa sinks to the bottom and takes up the life of the adult. The 

 stage with five tentacles is called by Semon the Pentacula stage. 



The development of the genital organs has been observed by 

 Clark in Synapta vivipara. In this species the eggs, when ripe, 

 burst the covering of the ovarian tube and fall into, the body-cavity 



