ANATOMICAL NOTES ON THE ' COMET ' OF LIXCKIA MULTIFOIiA, LAMARCK. 71 



principal arm possesses a larger tube, larger ampullae and larger tube-feet 

 than those of all others, but there is nothing unique in their structure. 



One of the characteristics of a ' comet ' is the small size of the innermost 

 ambulacral plate of the principal arm. Fig. 7 represents the proximal end 

 of the vertebral ridge of a normal arm whose distal part has, however, recent- 

 ly fallen and Fig. 6 that of a fallen arm which has built the younger ' comet.' 

 In the former the innermost plate which is, as Ludwig^*^^ noticed in Aatropeden 

 and Asteracanfhion, formed through a fusion of two plates, is the largest 

 among the ambulacral plates, while in the latter it is incomparably smaller 

 than many succeeding plates, but is nearly equal to those of the young 

 arms. The same relations are observed in the other ' comet.' This fact 

 connected with the corresponding slight development of the stomach retractor 

 muscules, mentioned before, proves that in these ' comets ' the innermost 

 ambulacral plate, (consisting of two fused plates), of the principal arm has 

 been formed secondarily, after the same arm was fallen. It follows also that 

 the same plate is almost of the same age with those of the young arms. In 

 other respects, the vertebral ridge of the principal arm is exactly similar to 

 that of a developed normal arm ; and those of the young arras to those of a 

 normal young. 



Small patches of the generative organ are widely distributed on the 

 lateral perivisceral wall of the principal arm. Cells contained in them are 

 yet immature so that sex can hardly be determined in thin sections, especially 

 in the younger* ' comet', but nearer the disc the patches are comparatively 

 larger as in normal forms. The patches are, however, j'^et invisible in the 

 young arms in these ' comets ' even with a hand lens, magnifying into ten 

 times. 



Some normal speciemens have an arm or arms with a constriction at a 

 certain distance from the disc, and that constriction is sometimes very re- 

 markable as shown in Fig. 2, A. In such a case the integumental wall as 

 well as the perivisceral cavity of the arm is somewhat constricted (Fig. 10) 

 so to produce some pairs of the ampullae considerably reduced in size 



* It should always he kept in minrl that I use the term younger in respect to the develope- 

 ment of the small arms as well as the internal organs, but not of the generative cells. 



