CHAPTER li 



Metamorphosis 



a. Arbacia Punctulata. Plate VI 



After the 3 or 4 day old pluteus, raised in the laboratory, is fed J\fitz- 

 schias, it increases in size, and the anal arms grow to about 600 (j. (from 

 400 [jl) when a week old. Then little knobs appear toward the base on 

 the pluteus, which, by the eleventh day have grown out into a new 

 pair of arms extending backward, the postero-lateral (Mortensen) or 

 ventral-lateral (Brooks) (Plate VI, Photograph 3). These arms have 

 very red tips owing to accumulation of red pigment bodies, and they 

 have much longer and stronger cilia than the other arms. These arms 

 grow longer and another pair of knobs appears between the original 

 anal arms and the new red-tipped arms (2 to 3 weeks) ; these become 

 the postero-dorsal (Mortensen) or dorsal-lateral (Brooks) arms (Photo- 

 graph 4). These arms have fenestrate rods like the anal arms, unlike 

 the red-tipped arms which have solid rods. All the arms grow much 

 longer, and the animal is easily visible to the naked eye, looking like 

 a small spider (Photograph 5). The animal tumbles about on the tips 

 of its arms and also swims by means of its cilia. The arms are variable 

 in length individually and relatively to each other. They are very 

 fragile and are easily broken off when the animal bumps into something 

 or when transferred to another dish. They have great regenerative 

 capacity, the arms growing out again when broken off. One pluteus 

 from which I had cut off the red-tipped arm about halfway down, had 

 completely regenerated it together with the red pigment in five days, 

 so that it looked exactly like its mate. When three or four weeks old, 

 two pairs of tubular processes appear, two dorsal, two ventral, the 

 auricular lobes (Photograph 6). Tho more pairs of arms arise in the 

 head end, the antero-lateral and the antero-dorsal, so that there are 

 now six pairs of arms, three long pairs and three shorter pairs. The 

 body of the adult Arbacia is now seen as a yellowish green mass in the 

 pluteus, the dark area in Photograph 6 and thereafter. There are areas 

 of dark red pigment on the surface of the body. The young adult is 

 formed in the body of the pluteus and grows at the expense of the 

 pluteus. 



