1909.] 



AN ABNORMAL ECHINOID. 



717 



of area 3 are discontinued after they have wrapped round the 

 adapical end of ambulacrum III, and their place is taken by a 

 single median series of heptagonal and hexagonal plates, so that 

 a compound area of three columns of plates abuts on the hyper- 

 trophied madreporite. 



The structure of the ambulacra at and towards the ends of the 

 truncated areas is shown in text-figures 229 and 230. The pori- 

 ferous plates are seen to curve round the extremities of the zones 

 with considerable regularity, leaving a regularly rounded outline 

 rather than an a,brupt bi^eak in the course of the growth of the 

 area. 



Text-fig. 229. Text-fig. 230. 



Abnormal ambulacra oi AmhJijpneiistes. 



Counting from the peristome there are about 25 sets of plates 

 in ambulacrum III, and about 28 in V. In a normal ambulacrum 

 there are about 56. 



The abnormal development, or rather, lack of development, in 

 this specimen is quite similar to that noted by Messrs. Ritchie 

 and Mcintosh in Echinus esculentus, except that the retardation 

 of ambulacral growth has aft'ected two areas instead of one only. 

 The development of fresh ambulacral plates seems to have been 

 checked at a dilFerent period in the life of the animal in the case 

 of each ambulacrum, and no such corresponding irregularities at 

 about the same region of the other areas are to be found in the 

 Ambli/pneustes as there were in the case of the Echinus. Thus 

 the hypothesis of a uniform and temporary wound or disease 

 affecting the growth of new plates round the apical system, which 

 could account for the latter's abnormalities, does not seem 

 tenable in this case. Moreover, the great irregularity in the 

 numbers and proportions of the plates of the apical system seem 

 to point to a more radical morbidity than is compatible with the 

 idea of mei^e local injury. 



It seems probable that the two missing ocular plates of the 



