1 88 BULLETIN OF THE BUREAU OF FISHERIES. 



third, fourth, and fifth segments diminish very rapidly, the fifth being only one-third 

 the width of the second. The genital segment is wider than the fifth segment, while 

 the abdomen segments are considerably narrower. 



The maxillipeds have a rather slender basal joint, with an enlargement on the inner 

 margin just above the spine and dose to the base of the terminal joint. 



Total length of female, 1.25 to 1.50 mm.; of male, i to 1.15 mm. Carapace of 

 female 0.32 mm. long, 0.20 mm. wide; of male 0.28 mm. long, 0.24 mm. wide. Anal 

 laminae and setae 0.20 to 0.30 mm. long. 



Color of both sexes like that of yellowish cartilage, the digestive tract black, the 

 tripartite eye a bluish-purple. 



THE INTERNAL STRUCTURE OF THE FOURTH COPEPODID LARVA. 



Circtdation. — The most conspicuous part of the internal mechanism (fig. 62-67, pl- 

 xiii) of the living larva is the digestive tube, which runs straight through the center 

 of the body. Its contents are jet black in color and reach back into the genital segment, 

 the portion of the intestine behind that, together with the rectum, being transparent 

 and colorless. 



Every little while a small portion of this black material, which is the partly digested 

 blood from the gills of the host, is separated from the rest, rolled up into a rounded 

 mass, and passed back into the rectum, from which it is soon ejected. 



The only circulation in the larva is produced by these movements of the digestive 

 canal. The anterior end of the stomach is fastened to the dorsal wall of the head by 

 three muscles, whose contraction draws the entire digestive tube forward. It is then 

 drawn back again by muscles cormected with the rectum and the posterior end of the 

 intestine. 



In addition to these forward and backward movements there is also a peristaltic 

 wave of contraction, which staTts at the anterior end of the stomach at the same time 

 that the latter is drawn forward. This wave travels backward and reaches the rectum 

 at about the time the forward movement of the whole digestive canal ceases. Accord- 

 ingly when the backward movement of the canal begins the peristaltic contraction is 

 reversed and passes fonvard again. The combination of these two kinds of movements 

 produces corresponding impulses in the contents of the body cavity around the digestive 

 tube. But this is e\'idently more of a pulsation than a circulation, the body fluid simply 

 mo\nng back and forth longitudinally and transversely, without crossing from one side 

 to the other and with very little real mixing of the various portions. It may be noted 

 here that this same backward and forward movement of the digestive tube persists in 

 the mature female and constitutes her only method of respiration. 



The digestive canal. — ^The mouth is comparatively large and opens into a long and 

 narrow oesophagus {oc, fig. 62, pi. xiii) which extends back of the center of the cepha- 

 lothorax, the sphincter muscle at the opening into the stomach being exceptionally large. 

 The walls of the stomach are thick and the digestive cells {dc) project so far as to nearly 

 close the lumen. In this lan'a there is no distinction between stomach and intestine 

 or between intestine and rectum; it is all one tube tapering gradually backward. The 

 walls of the cesophagus are composed of an inner layer of pavement epithelium and a 

 very thin outer muscular layer. The latter is much thickened over the stomach and 

 intestine, where it produces the peristaltic movements just described. In place of the 

 pavement epithelium both stomach and intestine are lined with a glandular epithelium 



