NOETH AMEEICAX STAEFISHES. 95 



tube (PI. IX. Figs. 5, 6) between this plate and the following reticula- 

 tions, forming a part of the sides of the arms ; similar pores are found 

 arranged like the first row of pores in a line parallel to the longitudinal 

 axis of the arms. The other water-pores are ii-regularly placed over the 

 surface of the arms. 



Near the mouth the interambulacral plates come together in the angle 

 of the amis and form the mouth-papillae so called. The}' are readily 

 seen in a Starfish examined from the lower side (PI. IX. Fig. 5) when 

 denuded of spines. Seen from above (PI. IX. Fig. 6, an interior view), 

 the connecting plate between adjoining ambulacral systems is formed 

 by the rising of the outer edge of the plate (the outer pore not being 

 present) towards the limestone network formed by the junction of the 

 interambulacral imbricating pieces which constitute the framework of the 

 abactinal system. 



This structure is best seen in large specimens of A. vulgaris, in which 

 the alternate arrangement of the ambulacral plates commences at once at the 

 base of the arms, and where the interbrachial fold at the angle of the arms 

 is high and well set off from the pores left for the passage of water-tubes. 



The spines placed on the junction of the interambulacral plates (a", PI. 

 IX. Fig. 5) form the papillte (PI. IX. Fig. 6), near the actinal opening ; 

 they differ in no respect from the other spines. 



The arrangement of the pores in double rows (PI. IX. Figs. 5, 6) for 

 the passage of the ambulacral suckers is, as is Avell known, only due to 

 age, owing to the crowding of adjoining plates ; in large specimens there 

 is no trace of the original linear arrangement of the ambulacral poi-es 

 beyond the plates nearest the actinostome or at the extremity of 

 the arms. But while the ambulacral pores thus alternate, the plates 

 themselves extend entirely across from the median line to the interam- 

 bulacral plates ; they are wedge-shaped, the broad and pointed ends 

 of adjoining plates alternately extending to the median line of the arm 

 or to the edge of the interambulacral plates (PI. IX. Figs. 5, 6). See 

 a note on the fecundation of A. berylinus and A. vulgaris in Archives de 

 Zool. Exper., which suggests a plausible cause for the great number of 

 varieties of the genus Asterias. A. Forbesii (berylinus) extends from Hali- 

 fax, N. S., to Florida, while A. vulgaris (pallidus) has a more limited 

 southern range, and extends farther noi-th, from Labrador to Long Isl- 

 and Sound. lu Massachusetts Bay the two species are about equally 

 common. 



