of the Staj'-Jishes. 135 



formed in the skin. At the middle of the back tlicre was an 

 opening, just at the spot where the prominent orifice, in the form 

 of the neck of a bottle, occurs in the adult animal. In the 

 middle of the ventral plane the buccal orifice was observed, sur- 

 rounded by five calcareous pieces, and closed by a pretty strong 

 membrane. The stomach was distinctly formed, and suiroimd- 

 ing it the aquiferous canal already mentioned was observed, 

 from which five canals passed to the five rays. From each canal 

 lateral canals issued, which terminated in the pedal vesicles. In 

 this specimen there were three pairs of ambulacra. 



On the 12th of August we again examined some specimens. 

 The larval mouth still existed, but a portion of the intestinal 

 canal of the larva had already begun to disappear. On the back, 

 which was tolerably convex, the madreporic plate was found in 

 an interradial space. Four pairs of ambulacra were formed. 

 The skeleton was then pretty well developed. The membrane 

 which closed the buccal aperture still existed; it docs not dis- 

 appear until afterwards, so that the true mouth is only formed 

 after the young animal has quitted its mother. 



If we glance at the observations which we have made upon 

 the development of Pteraster militai-is, we shall see that the 

 embryo passes through three stages, — the embryonic state, that 

 of the larva, and that of the Echinoderm, all of which are passed 

 through whilst the embryo is enclosed in the maternal cavity. 

 On comparing this, in the first place, with the Star-fishes whose 

 larvae are furnished with claviform prehensile organs, but never- 

 theless, as regards the development of the other internal organs, 

 are still in the embryonic state, and then with those which have 

 special larval organs which subsequently disappear, we shall find 

 that the development of Pteraster nnlitaris difF'.rs in several 

 points from that of those larvie. In Pteraster, the form of the 

 larva is oval, without prehensile arms, and it is furnished with a 

 buccal orifice, and with a long and narrow intestinal canal, which 

 terminates by an anal orifice. Both the mouth and the re- 

 mainder of the intestinal canal disappear by absorption, but only 

 some time after the true Echinoderm has made its appearance. 

 Even in the larva the rudiment of the stomach of the future 

 Star-fish is perceived ; this, howev(ir, does not arrive at its per- 

 fect development until the radiate form has made its appearance. 

 For some time the larval mouth and the intestinal canal and 

 anal opening are still observed^ for it is only at a later period 

 of development that all these attributes disappear, after the 

 formation of a mouth in the centre of the ventral surface and an 

 anal orifice on the back. 



In Bipinnaria asterigera the oesophagus of the larva enters 

 into the back of the Star-fish excentrically and interradially. 



