26 Prof. Miiller on the Development 



those of the sac, but the blood must also penetrate into the ca- 

 vity of the intus-suscepted portion as far as its blind end. 



There are two intestinal vessels, as in other Holothuria ; both 

 are very capacious, and they lie one on the free side of the intes- 

 tine, the other along the line of attachment of the mesentery. 



It is always the former which is connected with the molluski- 

 gerous sac, and the mode of connexion is always the same. The 

 place at which the attachment takes place is anteriorly, a short 

 distance (at the most 1^ inch) behind the muscular stomach 

 possessed by the Synaptce in common with many Dendrochirota. 

 If there be two sacs, they are attached one behind the other in 

 just the same manner. 



The two intestinal vessels present under the microscope violent 

 undulating contractions of their walls, such as may be seen in 

 the intestinal vessels and vascular plexuses of Holothuria tubulosa. 



The undulatory movement is continued on to the vascular 

 branch which invests the knob of the molluskigerous sac. There 

 is no ciliary motion in the interior of the intestinal vessels, the 

 blood-corpuscles rolling hither and thither only in consequence 

 of the undulatory contractions ; on the other hand, the vessels 

 and the intestinal walls are ciliated upon their outer surface, and 

 this ciliary investment is continued on to the outer surface of the 

 vascular branch which invests the "knob," but it stops short 

 where the wall of the vessel unites with that of the sac, and the 

 latter has no cilia upon its outer surface. In this it is distin- 

 guished from the ordinary ovarial sacs of the Synapta, for these 

 are externally ciliated. 



The mollusks are not produced within that portion of the sac 

 which contains the intus-susception, but in the other part. In 

 this more capacious portion, both the male and the female ele- 

 ments of the mollusks, and subsequently the mollusks themselves, 

 are found. The organ which contains the female elements may 

 be called an ovarium, and that which contains the male elements 

 may be called a testis, but in structure they have no resemblance 

 whatever to ordinary testes or ovaria; their products however are 

 identical with the ova and spermatozoa of other animals. Both 

 ovarium and sperm-sacs lie free within the molluskigerous sac, 

 and are in no way attached to it. The ovary lies next to the 

 intus-susception, and thereupon follow the numerous sperm-sacs ; 

 but before describing these, it will be necessary to enter into the 

 minute structure of the walls of the molluskigerous sac. 



The outer layer of this sac and the inner of the intus-suscep- 

 tion consists of perpendicular elongated (palisaden-fbrmig) cells, 

 from whose contents the green colour of the green portion arises, 

 for they contain yellowish granules which in the green part are 



