418 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



The anterior or optic enlargement is continuous on each side with the ganglia of the eye stalks. 

 Its two halves are united by transverse fibers. The lateral enlargement is markedly kidney-shaped, 

 and from its hilus there issues a complex system of fibers. A great part of these fibers issue from 

 the ganglion cells inclosing the lateral ball, pass out in a bundle to the middle of the brain, 

 and thence up to the optic ganglia, apparently without crossing. The fibrous substance of the 

 lateral enlargement has a pyramidal structure that is, its tissue is permeated with pyramidal blocks 

 of a denser substance which stains faintly with carmine. The apices of these pyramids point 

 toward the center of the ball. Below the lateral enlargement and nearer the middle line there is 

 the antennular mass from which the auditory nerve issues. This is bilobed and has the same gen- 

 eral relations as in the first larva (PI. LV, Fig. 216, //.). The antenual enlargement, which is 

 closely connected with the latter, is now a much more conspicuous mass than in the earlier stage. 

 From it issue the auteuual nerve and the uesophageal commissures. 



The alimentary tract has undergone very important changes, of which mention was made in 

 Section i. What corresponds to the foregut of the larva comprises that portion of the tract from 

 the mouth to the duct of the gastric glands. It is divisible into a straight and vertical portion, 

 the -lesophagus proper and a masticatory stomach. The latter consists of two parts, an anterior 

 section contkmous with the oesophagus, and probably corresponding with the gastric mill, 

 and a pyloric portion or strainer. In transverse section the cardiac section appears circu- 

 lar and the walls are rather thin and slightly folded. As this passes into the pyloric division the floor 

 of the tract rises into a broad tongue-shaped process, which is surmounted by a particular strainer 

 of hairs. This median fold is continuous throughout the pyloric division, where it narrows into a 

 crest. The lateral walls, which are greatly thickened and studded with hairs, approach each other 

 so that only a small lumen is left, through which the food is strained as it passes over the net- 

 work of hairs. These two sections of the stomach correspond to portions of the tract marked Ms., 

 Fig. 19G. They are of epiblastic origin, and in passing to the inidgut the epithelium suddenly 

 changes as it does in the larva. 



The ducts of the three glands unite on each side and thus form two common ducts. The 

 epithelia of the gastric c;eca of the ducts and midgut are directly continuous and pass gradually 

 into each other. They consist of a delicate layer of connective tissue which forms a capsular in- 

 vestment and the large columnar cells already described. In the "liver" many of these cells are 

 in the process of active secretion, and as a result of this activity the lumen of the gland is filled 

 with a coagulated liquid. The secreting cell at this period swells out to two or three times its 

 former size and has the form of a distended bladder projecting into the cavity of the gland. The 

 contents of the cell consist of a light granular fluid and when the cell breaks down this is dis- 

 charged into the lumen of the gland. 



Just how much of the wall of the alimentary tract behind the glandular ducts is of endodi-r- 

 mic origin it is impossible to say, since from the first there is no sharp morphological boundary 

 between the hiudgut and meseuteron.' But it is certain that only a very small portion of it can 

 arise in this way, and that the eudoderm of the larva goes mainly, if not exclusively, to form the 

 lining of the gastric caeca. 



The heart is very much compressed from above downward as in the adult. The walls of blood 

 vessels consist of a single layer of cells which secrete a homogeneous limiting membrane. As 

 in the early larva, the heart and pericardium are screened from the digestive tract and other organs 

 by a horizontal membrane. The reproductive organs are still in the rudimentary condition 

 already described. 



V. NOTES ON THE SEGMENTATION OF CRUSTACEA.* 



Under this head I will add a few notes' upon the segmentation of the egg of a number of 

 Decapods, which have been studied chiefly for the sake of comparison with Alpheus. 



Alpheus xuiilci/i and A. heterochelis are both typical examples of centrolecithal segmentation 

 with the formation of yolk pyramids. They possess a massive food yolk and hatch as mysis-like 



* The concluding sections of this memoir were written after the lapse of a long interval, during which time it was 

 not possible to refer to niy earlier manuscript. This will account for any unnecessary reiteration of facts, or for any 

 inconsistencies iu statement which I have failed to eliminate. 



