10 BULLETIN OF THE 



but a notch where it is later situated appears in a genital long before 

 the calcification of the stone canal has appeared. The madreporic 

 opening, as shown by Ludwig's figures of Asterina, lies to the left of 

 the first genital. 



The stellate form of the starfish is brought about by the growth of 

 the plates of the arms, by which the terminals or first plates to appear 

 in the body are pushed out to the extremity of the rays. Among the 

 most important of these plates, on the abactinal side of the body, are 

 the dorsals, d, and the marginals or laterals, m. On the actinal side, 

 the ambulacrals, am, and the interambulacrals, ad, accomplish a similar 

 result. All of these plates follow in their growth a general law, viz. 

 that the new plates are formed between those which have already 

 appeared and the terminals. A somewhat similar law holds in regard 

 to the formation of the plates on the disk between the dorsocentral and 

 the first radial or median dorsal. In the case of the latter plates (dor- 

 sals, d), however, those nearest the dorsocentral are the last to form. 

 The first radial, d, or first dorsal, is therefore a point of departure, on 

 either side of which calcifications appear. On the arms the first formed 

 plates are nearest the first dorsal. In the plates formed in Amphiura, 

 the primary radials, or radialia, have a similar relationship. 



In the general development of the water-vascular system, it may be 

 noticed that the extensions, /, from the medial water tube, which form 

 the ampullae, are formed before the ambulacral rafters which ultimately 

 separate them. The feet, /, are at first destitute of suckers, and are 

 arranged in two rows (PI. III. fig. 3) in each arm, one row on each side 

 of the middle line. A terminal tentacle (PI. III. fig. 4, tta) was ob- 

 served in a young starfish, in which there are but two pairs of lateral 

 feet, and no ambulacral rafters. Even in this early stage the eye spot 

 is well developed on the terminal tentacle, tta. 



The madreporic tube forms a conspicuous object in early starfish 

 larvae, and passes by successive change into the madreporic canal of the 

 older specimens. The calcification of the canal was observed in early 

 specimens before the formation of the madreporic sieve, which is found 

 superficially on the abactinal surface of the adult. The cribriform mad- 

 reporic plate is of comparatively late formation in the growth of the 

 starfish. None of the important primary plates of the young starfish 

 form by a constriction from others previously formed, but each kind of 

 plate originates from its own calcification. Plates which have originated 

 from two centres rarely consolidate, although their connection may be 

 of a very intimate nature. 



