228 



H. papillifera, and H. spinifera (Mortensen, 1937, 1938). In Caudina chilensis 

 the gastrula develops during 33 hrs (Inaba, 1930). According to Mortensen 

 (1937, 1938), in Ophiodesoma grisea, Synaptula vittata, Holothuria spinifera, 

 and H. papillifera, embryos hatch at this stage. 



In holothurians, gastrulation proceeds through invagination. Before that, 

 the blastula slightly stretches along the animal-vegetal axis and its vegetal 

 pole flattens. Ohshima (1921) in Cucumaria echinata and Inaba (1930) in 

 Caudina chilensis have reported immigration of a large number of mesenchy- 

 mal cells before the initiation of gastrulation. In other species immigration of 

 the mesenchymal cells generally occurs from the tip of the archenteron, after 

 it reaches the middle of the gastrula (Figure 176). Later, the archenteron 

 bends to the future ventral side. Then the mouth opens, a through gut appears, 

 and the blastopore becomes the anus. Division of the gut into sections is not 

 yet discernible. In species in which development occurs through the 

 planktotrophic auricularia stage, a dipleurula is formed. 



Dipleurula 



Formation of the dipleurula begins when the gastrula changes shape. It 

 bends on the dorsal side and a large preoral depression forms on the ventral 

 side. The larva acquires a beanlike shape. The formed preoral lobe slightly 

 overhangs the oral depression.. Formation of the anal lobe involves displace- 

 ment of the anus and turning of the lower end of the intestine. The anus is 

 now situated on the ventral side in the middle of the anal lobe. Cilia covering 

 the gastrula disappear altogether, except on the edge of the preoral depres- 

 sion and lobes. A single ciliated band is formed. 



Soon the gut divides into sections; esophagus, stomach and hind gut 

 (Figure 177). The epithelium of the gut is provided with cilia. At this stage, 

 no lateral body processes arise. Rudiments of the coelom are present in the 

 form of clusters of mesenchymal cells. 



Auricularia 



In most species the early auricularia develops in two days. Such are the 

 auriculariae of Ophiodesoma grisea, Holothuria scabra, H. spinifera, H. 

 impatiens, and H. pardalis (Mortensen, 1937, 1938). Lateral processes form, 

 at this stage. In the late auricularia the lateral processes, up to six pairs, are 

 well developed but not equal in length. Sometimes one or two pairs are 

 especially prominent in their size (Figure 178). In the larvae of holothurians 

 these processes are designated as follows: the anterodorsal and mediodorsal 

 lie above the mouth opening, while the preoral processes lie at the level of 

 the mouth opening. In the lower part of the larvae there are postoral, 

 posterodorsal and posterolateral processes. 



