MUSEUM OF COMPARATIVE ZOOLOGY. 7 



found not to be parallel with tlic plane in uhich a line drawn through 

 the tips of the radial water tubes, or the first-formed ambulacral jjlates 

 lie. As absorption of tlie brachiolaria goes on, however, these planes 

 get more and more nearly parallel, so that the two surfaces equidistant 

 from the axis are equidistant bom each other at all points. 



Each of the five small cuU-de-sac, rir, from the water tube on the 

 ambulacral side of the young starfish forms a radial water tuije of 

 the starfish; and if a line be drawn from the tip through its middle to 

 the centre of the mouth, it miglit be thought to indicate the line of the 

 ray. In the same way, if a line be drawn from each of the notches 

 in the margin of the young starfish (I'l. 1. fig. 3) through the centre 

 of tiie mouth, it might be thought to indicate an interradius of the ab- 

 actinal side. The radius and interradius thus formed have in the adult 

 a definite relationship to each other. They do not coincide, as tliey 

 indicate entirely different regions of the voung starfish. If such lines 

 be projected in Plate I. fig. 3, it will be seen that there is a very great 

 variation in their relative distances from each other. This difference is 

 in part due to the oblii^uity of the two planes of actinal and abactinal 

 regions of the starfish. 



The first addition, on the abactinal side, to the ten plates which form 

 the two Us of the early stages, is a small calcareous nodule, situated 

 within the smaller U near the fifth genital (I'l. I. fig. '!). Tliis nodule 

 is the beginning of the dorsoccntral, dc, and in the subsequent growth 

 of the fifth terminal towards the first genital by the absorption of the 

 brachiolaria, and the consequent reduction in size of the brachiolarian 

 notch, it is brought to occupy the centre of the abactinal region of the 

 starfish. The anus of the brachiolaria, which is the blastopore of the 

 gastrula, is situated quite a di.-^tnnce from this plate, and not near it as 

 recorded in Astcri^ua. 



The growth of the fifth termmal, t^, towards the first genital, g^, which 

 from its vicinity to the madreporic opening is called the madreporic 

 body, is brought about by an absorption of the brachiolaria, and the 

 reduction in width of the notch as stated above. Before the complete 

 closure of the brachiolarian notch takes place, however, the terminals 

 have grown so large that marginal notches corresponding to interradii 

 have formed between them (PL I. fig. 2) on the rim of the body, and 

 an ajiproach to the stellate form begins to be visible. The increasing 

 growth of the rods of the forming star adds so much weight to the 

 brachiolaria that it sinks to the bottom of the aquarium in which the 

 animal is confined. The eleven plates of the abactinal region of the 



