I. Morphological and Physiological Works of a General Character. 7 



discharges a chrornatogenic function in addition to preparing- the digestive 

 ferments. 



Fewkes ( 2 ) has come to the conclusion that the mouth plates of two representa- 

 tive Starfishes. Asterius and Asterina, are built essentially on the same plan, and 

 that Viguier's division of the group into Starfishes with ambulacral and adambu- 

 lacral mouths respectively is not a natural one. These mouth plates are homo- 

 logous in both cases with ambulacrals and may be regarded as the distal extre- 

 mities of the ambulacrals and homologous with so-called adambulacrals, which 

 are themselves simply modified ambulacrals . In both genera the adambulacrals 

 originate between the outer ends of the ambulacral rafters with which they are 

 serially homologous. They simply differ from the ambulacrals in the time of their 

 development and limitation to the border of the ray. The presence of spines upon 

 them does not interfere with this homology. The actinal skeleton of the Starfish 

 arm consists of a number of somites, the alternate members of which are calcified 

 so as to approach each other and to join on the middle line of the arm as the 

 ambulacral rafters, while others remain on the periphery and form the adambu- 

 lacrals. The case is reversed in the Urchins, for the ambulacral rafters are 

 wanting or only represented by the auricles, while the alternating adambulacrals 

 have grown to join at the middle line outside the water-vascular system. Hence, 

 like Ludwig, F. regards the so-called ambulacrals of the Urchin as representing 

 the adambulacrals of the Starfish ; while the interambulacrals of the Urchin corre- 

 spond to the marginals of the Starfish, but those of adjacent rays remain in con- 

 tact and are not separated by interbrachials, as in Starfishes. The ray of a Star- 

 fish is thus composed of a number of rings each consisting of two parts one 

 enclosing the water-vascular and nervous system, the other the digestive, forming 

 the body cavity . In the primary somites the calcifications of this ideal ring in- 

 side the water-system are developed and join across it. In the alternating secon- 

 dary segments neither the portion of the calcareous ring above the water-vessel 

 nor that below it is calcified, but only those on either side. In the Urchins these 

 form outside plates which join on the middle line. Hence, while the adambulacrals 

 of the Starfish are homologous with the ambulacrals of the Urchin, the exact com- 

 parison should be with different regions of the ring in the two groups, for the 

 ambulacrals of the Starfish belong to the primary, and those of the Urchin to the 

 secondary segments. 



Fewkes ( 3 ) points out that both in Stellerids and in Urchins the spines of Ili< 

 early calcareous plates are relatively much larger than those formed later, and also 

 differ somewhat from them in shape. Those of Asterids and Echinids are formed 

 before the end of the Brachiolarian and Pluteus stages, while they also occur in 

 Asterina and other Starfishes with a direct development; but there are none in 

 the viviparous Amphiura which has no free Pluteus stage. The peculiar fan- 

 shaped spines of the young Starfish (nectospines) may be the survivals of a 

 swimming organ like that of OpMopleron, which has become functionless in the 

 modifications of the body. Two kinds of spines are recognisable in the Starfish : 

 1. those which are formed as papillae from the plates, 2. those formed from 

 separate centres of calcification which are subsequently articulated with the plates. 

 The first group includes the fan-shaped nectospines and also others of more uniform 

 diameter. The first formed spatulate spines of ArJbacia and the trifid primary 

 spines of Echinarachnius are homologous with the ten nectospines on the terminals 

 of Asterias, having the same relation to the radii. The young Arbacia has been 

 obtained free swimming, after absorption of its Pluteus, and is thought to resemble 

 the ancestral form (Archiarbacia) from which have sprung the stellate and echinoid 

 groups of Echinoderms. This had a discoidal body, on the periphery of which 



