•6o4 



ECHINODERMATA 



verted into a tube called the " archenteron " (Fig. 282, C, arch), 

 which is the rudiment of both the alimentary canal and the 

 coelom of the adult. This tube communicates with the exterior, 

 in virtue of its mode of formation, by a single opening which is 

 called the " blastopore," which becomes the anus of the later 

 larva and adult. Whilst the gastrula stage is being acquired, 

 the blastocoel or primary body-cavity is invaded by wandering 



cells budded from the 

 wall of the archenteron 

 (Fig. 282,A, B,C,??i^s). 

 These cells, which are 

 called " mesenchyme," 

 are the formative cells 

 of the skeleton, con- 

 nective tissue, and 

 wandering cells of the 

 adult. When the larva 

 has a skeleton they are 

 formed very early, aris- 

 ing in the young blas- 

 tula stage (Ophiu- 

 roidea) or in the stage 

 immediately before the 

 formation of the arch- 

 enteron (Echinoidea, 

 ¥10,. 2%2.—Echinii^ esculentus. A, optical section of j'j^q.^ 282, A, B) and 

 livint; hlastula. B, section of preserved blastula. ^' . '■,'■,■,, 

 The network of strings in the interior is the result SCCrcting the skeleton, 

 of tlie coagulation of the albuminous fluid. C, "\y}jgi^ ^]^q larva is 

 section of gastrula. arch, Archenteron ; mes, mes- 

 enchyme cells, attached by protoplasmic strands to dcVOid of a skeleton 



the wall of the embryo, x 150. (Asteroidea and Holo- 



thuroidea), the mesenchyme usually does not appear till the 

 gastrula is fully formed. 



The gastrula stage is reached in twenty to thirty-six hours. 

 Then one side of the larva becomes concave, and the cilia Ijecome 

 restricted to a thick band surrounding this area. In this way 

 is formed the rudiment of the longitudinal hand of cilia, which is 

 the organ of locomotion throughout the larval life. At the 

 apex of the archenteron a thin-walled vesicle is formed, which 

 soon becomes divided off from the rest. This vesicle, which 

 almost immediately divides into two sacs, right and left, is the 



