670 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxv. 



Most of these latter fibers, as well as those of the long-itudinal muscles, 

 are one-celled, and the central nucleus is plainly visible, with one or 

 more long- fibrous portions extending- from either end. These muscle 

 fibers are strikingly- like those elsewhere described^ in the veliger 

 larvie of nudibranchs and the pilidium larvae of neraerteans, and in all 

 probabilit}' the fibrous portion is developed in a similar manner by a 

 fibrillar rearrangement of the protoplasm of the original cell. These 

 muscles, both longitudinal and dorso-ventral, contract somewhat 

 rh3'thmically and drive the blood from the space between them around 

 the cloaca out into the posterior end past the anal papillae into the 

 peripheral spaces along the sides of the abdomen. 



The second muscle to aid in the pulsation takes the form of a trans- 

 verse dorso-ventral band or curtain lying at the posterior border of 

 the last thoracic segment in just the position occupied later b}^ the 

 posterior wall of the heart {/i.. fig. 9). This is attached to the side 

 walls of the sinus between thorax and abdomen, but hangs loosely 

 elsewhere, and on contraction produces two movements, one a pulling 

 together of the side walls and the other a l)ackward and forward move- 

 ment of the center of the curtain. Both motions aid the streaming of 

 the blood. Besides these muscles, which are directly concerned in cir- 

 culation, there are others in the walls of the stomach and intestine, 

 on the dorsal and ventral walls of the thorax, and even in the basipods 

 of the legs, that must assist the process considerably. The internal 

 tissues are so loosel}" put together and there is so much free communi- 

 cation between the various parts of the body that a vigorous contrac- 

 tion of any set of muscles, e. g., those in the basipods of the legs 

 during swimming, must produce more or less of a flow of blood in the 

 immediate vicinity. 



The skin is so very transparent that even the transverse striation of 

 the muscles can be readily seen through it, so that these larvae afford 

 one of the best objects for a study of crustacean musculature that 

 could be imagined. 



The skin l)eing so thin and the blood circulating everywhere freely 

 beneath it there is no difiiculty in bringing about integumental respira- 

 tion. Such respiration takes place more or less all over the body and 

 does not seem to be exclusively concentrated in any one region. I agree 

 with Claus when, correcting the statements of both J urine and Dana 

 and Herrick, he says that the abdomen has no more to do with respira- 

 tion than some other parts of the body and is not as useful in this 

 respect as the side flaps of the carapace. This does not mean that the 

 abdomen does not function physiologically as a respiratory organ; it 

 certainly does and its service is a very valuable one. But it does mean 

 that this is not the only respiratory organ and probably not even the 



'Activities of Mesenchyiiu' in Certain Larvfe, C. B. Wilson, Zool. Bull., II, No. 1, 

 p. 15. The Habits and Karly Development of Cerebratulus lacteus (Verrill), C. B. 

 Wilson, (Jnart. Jour. Mic. Sei., new ser., XLTII, p. 97. 



