no THE AMERICAN ARBACIA 



where the mouth is formed, making the gut complete (about 1 7 hours 

 after fertihzation). The original in-pocketing remains as the anus. 

 Meanwhile (about 16 hours) the skeleton has appeared as a pair of 

 triradiate spicules, one on each side of the (incomplete) gut. During 

 this period there has been no appreciable increase in the size of the 

 organism over that of the egg (without the fertilization membrane), 

 and one would not expect an increase before the alimentary canal is 

 complete and it can take in food from the outside. Then growth occurs 

 and further differentiation. The axis of the larva changes, giving the 

 "prism" stage, and one of the prongs of the triradiate spicules elongates 

 (dark field photograph, Plate IV, Photograph 2). With further 

 elongation, these become rods on each side of the gut, and the early 

 pluteus is formed, at first without arms (Photograph 4, about 20 hours 

 after fertilization). At about this time, the large red pigment spots 

 begin to appear. The arms then grow out, increasing in length with 

 time. The pluteus is quite well formed a day after fertilization, and 

 increases in size during the next two or three days (Plate IV, Photo- 

 graphs 5-9). 



b. The Pluteus, Description. Plate IV 



The pluteus which we are accustomed to see in our cultures three or 

 four days after fertilization is roughly triangular in shape, swimming 

 with its arms forward, and its pointed end behind, by means of cilia 

 which cover the surface of the body. There are two pairs of arms, a 

 shorter pair on the dorsal side near the mouth, the oral or dorsal arms; 

 and a much longer pair on the ventral side, the anal or ventral arms 

 which may measure 400 [j. from base to tip (Plate IV, Photographs 7-9). 

 The skeleton consists or rods running into the arms, thinner ones 

 into the oral arms, and thicker ones into the anal arms; these long rods 

 meet at the base in a heavy spiny mass. There is also a transverse 

 connecting rod. The rods in the long anal arms of Arbacia are not 

 solid, but fenestrate or ladder-like. Many other sea urchins have this 

 same type of skeleton in their long arms, e.g., Tripneustes, Sphaerechinus, 

 and Echinarachnius (sand dollar). In other sea urchins, the skeleton of 

 the anal arm is a solid rod, e.g., Lytechinus, Psammechinus, Paracentrotus, 

 Strongylocentrotus. These two types of arm skeleton have been of great 

 value in hybridizing experiments, in determining maternal and paternal 

 inheritance. In Arbacia, another pair of arms which come in later, the 

 postero-dorsal arms, are also fenestrate. According to Fell (1948), the 

 fenestrate rods represent a primary structure since they are found in 

 the larvae of the more primitive forms. 



