10 Prof. Muller on the Anatomy and Development 



very small cells are imbedded, is seen lying like a mantle upon 

 the dorsal side of the stomach. 



The sac becomes developed into a rosette of five cseca, the first 

 foundation of the water- vascular system. 



The mantle-like mass curves over and covers in the stomach 

 and foundation of the tentacles like a cap, widely open below. 

 The dorsal pore becomes invested by it, and it extends round 

 the anus ; but the oesophagus remains outside it (Diag. XL). 



A crest or elevation now appears on the mantle-like mass, and 

 runs obliquely over it in a curved line, whose ends become even- 

 tually united. It then forms the margin of the starfish. 



What lies beneath this thickened margin belongs to the dor- 

 sum of the starfish, what lies above it to its ventral surface. 



The young starfish now attains a diameter of |th of a line, 

 becomes slightly pentagonal, and retains only a narrow connexion 

 with the Bipinnaria. 



The digestive canal, and with it the rosette-like rudiment of 

 the water-vascular system, becomes turned so as to present the 

 latter towards the ventral surface of the starfish, at that point 

 where its mouth is subsequently formed. The tube which con- 

 nected the rosette with the pore, which is now imbedded in the 

 dorsal surface of the starfish, receives a calcareous deposit and 

 becomes the sand-canal, while the " pore " is converted into the 

 madreporic plate. 



The oesophagus of the larva is obliterated, whilst its rectum 

 projects as an anal tube subcentrally from the dorsal surface of 

 the starfish (Diag. XIII.) . 



The slightest touch now separates the starfish from the larva 

 in which it was developed ; the former sinks to the bottom and 

 creeps by the aid of its newly-developed feet ; the latter swims 

 about as before for some time, but eventually perishes. 



In the Echinidse the process is essentially the same. An in- 

 ternal diverticulum of the integument of the larva is formed, but 

 from a somewhat different spot*, namely in front of the ciliated 

 fringe and on one side. It is connected with a vesicle which lies 

 close against the oesophagus, and from which the water-vascular 

 system is developed. 



At this place the shell of the Echinus subsequently makes its 



* It is remarkable that in the Asterid-larvae, while the development of 

 accessory ciliary processes, &c. takes place in the pre-trochal dorsal region, 

 the bud of the Echinoderm is developed from the post-trochal region. In 

 the Echinus-larvae we have just the reverse — the bud is developed from the 

 pre-trochal region ("below the lateral arch of the ciliated band," Muller), 

 while the processes, &c, as we have seen, are developed from the post- 

 trochal region. The Ophiure appear to present the same relations as the 

 Echinidae, though Prof. Muller has not been able to make out the point 

 with certainty. 



