xvi EXTERNAL CHARACTERS 453 



Having described a single species with some degree of fulness, 

 we must now give some account of the range of variation of 

 structure met with in the group. 



Number of Arms. In the overwhelming majority of Star- 

 fish the number of arms is 5, but deviations from this rule are 

 met with not only as individual variations, but as the character- 

 istics of species, genera, and even families. 



The number 5 is rarely diminished, but amongst a large 

 collection of specimens of Asterina gibbosa, belonging to the 

 author, some 4-rayed individuals are met with. One species of 

 Culcita, C. tetragona, is normally 4-rayed. 



On the other hand the number 5 is often exceeded. The 

 families Heliasteridae and Brisingidae are characterised by 

 possessing numerous (19-25) arms. In the normally 5 -rayed 

 family Asteriidae Pycnopodia has 2 2 arms ; and in the Solaster- 

 idae the genera Rhipidaster and Solaster are characterised by 

 possessing 8 and 11-15 arms respectively; whilst Korethraster 

 and Peribolaster have only 5. The common Starfish of the Gulf 

 of St. Lawrence, Asterias polaris, is 6 -rayed, whilst most of the 

 other species of the same genus are 5 -rayed, though 6 rays are 

 often met with as a variation. 



In some species the fact that the number of arms exceeds 5 

 seems to be connected with the power of multiplication by trans- 

 verse fission. Thus Ludwig 1 has shown that in Asterias tenuisjpina 

 the number of arms is usually 7, but sometimes 5, 6, or 8, and 

 that in most cases the arms are arranged in two groups one 

 consisting of small arms, the other of large. 



Shape. Apart from the varying number of arms, differences 

 in the shape of the Starfish are due to two circumstances : 



(1) The proportion of breadth to length of arm ; and 



(2) The amount of adhesion between adjacent arms. 



The adhesion can go so far that the animal acquires the 

 shape of a pentagonal disc. This is the case for instance in Cul- 

 cita. The fact that the body of this animal is really composed 

 of adherent arms is at once made clear when the coelom is opened. 

 This space is found to be divided up by inwardly projecting folds 

 called interradial septa, which are stiffened by calcareous deposits 

 and represent the conjoined adjacent walls of two arms. 



1 "Die Echinodermen des Golfes von Neapel," Fauna u. Flora G. von Neapel, 

 xxiv. Monogr. 1897, pp. 349-351. 



