390 MEMOIES OF THE NATIONAL ACADEMY OP SCIENCES. 



Next in point of size are a pair of large lateral balls, which appear liicluey-shaped in transverse 

 .section (PI. LV, Fig. 216, If.). Each is virtually segmented at the lower surface into two lobes (PI. 

 LVii, Fig. 242, l.f.). These lobes are closely united to each other, and by a pedicel or stalk of 

 libers to the lower posterior extremity of the anterior, optic mass. A third pair of fibrous masses 

 (PI. L.V, Figs. 215, 210, «/.) fuse with the anterior mass at the same point. Each of these balls 

 is also bilobed, and from them issue the fibers of the aiitennular nerves. (PI. LVII, Fig. 243, «. 

 au., also PI. LV, Figs. 212-214, a. o., naii.) The nerve of the first i)air of antennie consists of 

 cells and fibers, which pass to a mass of deeply staining cells {a. o.), the ear, and to the tissues of 

 the anteunular stalk. The fourth pair of fibrous masses (PI. LV, Figs. 217, 218, gf., also PI. lvii, 

 Fig. 243) are intimately associated with the last and with the common bridge of tissue (PI. LV, 

 Fig. 216, of.) which unites them all. From these arise the fibrous elements of the antennal nerves, 

 which supply the green gland and the tissues of the appendage (Fig. 216, n. ag.). From this 

 same region (Fig. 218,/o.) the commissures which surround the oesophagus and unite the brain to 

 the ventral nerve cord also originate (Fig. 220). These commissural bands meet immediately 

 behind and below the oesophagus, where they fuse (PI. LV, Figs. 222, ocm.) and join the ventral 

 chain of ganglia. This last consists of the ganglia of the remaining eighteen segments of the 

 body. Each ganglion is double and is made up of two fibrous balls, united by a transverse 

 commissure, and of a thick envelope of nerve cells. Longitudinal commissures of cells and 

 neive fibers unite the successive ganglia, which form a double chain. These relations are well 

 shown in Fig. 196 and by the horizontal section (PI. LVii, Fig. 243). The first six thoracic ganglia 

 are very closely crowded together (Fig. 196, g. 4-9) and form what is usually known as the 

 infra wsophageal ganglion (ganglia of mandibles, first and second maxillie, and first, second, and 

 third maxillipeds). The next five ganglia, g. 10-14, which are less closely crowded than the 

 preceding, belong to the five pairs of thoracic legs and their segments. The fiber balls of each 

 ganglion are pear-shaped masses, disposed vertically, with the large end of the pear turned toward 

 the base of the appendage. The abdominal ganglia are more widely separated and the longitud- 

 inal commissures are consequently more marked (Fig. 196, g. 15-20; see also the series of trans- 

 verse sections. Pis. LV, lvi). The nerves, always difficult to distinguish, owing to the close 

 similarity of their cells to those of the surrounding ectoderm, are best exemplified in the case of 

 the anteunular and antennal nerves already mentioned. 



The relations and course of the fibers, which are very complicated, are partially iudicated.in 

 some of the sections. There is a marked transverse commissure of fibers in the anterior half of 

 the large optic swelling (Fig. 213), and at its posterior extremity, where it fuses with the lateral 

 and antennal masses (PI. Liii, Fig. 198, _(//.). 



The optic stalks or lobes, bearing the compound eyes (PI. liv, Figs. 209, 210, and PI. lvii. 

 Figs. 239-242), consistof an irregular series of fibrous masses, in shape of a distorted letter L. 

 The angle of the letter L is continuous with the fibrous substance of the brain, while its shorter limb 

 proceeds to the compound eye and its longer forms a large swelling in the upper part of the stalk. 

 There is a nauplius eye (PI. liii. Fig. 197; PI. liv, Figs. 209, 210, oe.) borne on a median papilla, 

 which projects downward between the eye stalks. The details of the structure of the eyes are 

 given in Section ix. 



Thealimentary tract of the larva is a somewhat complicated structure, and the relations of its 

 parts are best undei'stood by reference to sections taken in more than one plane. We can recognize 

 five well-defined portions: the tesophagus, the masticatory stomach, the midgut, the hindgut or 

 intestine, and the appendages of the midgut. These are shown in asemidiagrammatic way in tlie 

 cut (Fig. 2), and the longitudinal section (PI. Liii, Fig. 190) and series of transver.se and horizontal 

 sections (Pis. lv-lvii) illustrate the stru(;tures in more detail. ' 



It is interesting at this point to compare the larva shown in Fig. 190 with the longitudinal 

 section of an advanced embryo (PI. XLViii, Pig. 168). In both we recognize the foregut, a tube bent 

 on itself, consisting of the tPsoi>hagus and masticatory stomach (Hi. s.). In the embryo the latter is 

 closed on the sitle of the food yolk. In both we also see a vertically directed fold of endoderm (/., 

 overlying mg^ In Fig. 190) and behind this the large lumen of the hindgut, which gradually tapers 

 into that of the narrow, tubular intestine. Between this fold on the one hand ard the stomach on 

 the other we find in the embryo an enormous space filled with yolk, which is partially walled in 



