MEMOIRS OF THR NATIONAL ACADEMY OF SCIENCES. 389 



it. Has natural selection, tlien, acted so far as to differentiate the species in more than one direc- 

 tion ! There are some facts which fovor the view that it has done so, but before the question cun 

 1)6 definitely settled we must determine more ])recisel.v how far intermediate or aberrant forms 

 rei)resent phases of tiie individual and of the race. It is not probable that we are here dealing 

 with the hybrids between two originally distinct species. 



Part Second, 

 the development of alpheus. 



1. STRUCTURE OP THE LARVA. 



(PI. XLix, Via. I'-t- P^- Lni, Kig. 196. Pis. i.iv-Lvii.) 



These studies in the embryology of Alidieus begin with the growth of the ovarian egg and the 

 early i)hases of segmentation and extend to tiie larval and adult periods. In order that the prog- 

 ress of development may bo followed in the light of the structure wiiich the embryo finally attnins, 

 we will start with a general survey of the anatomy of the first larva of Alphcus saiikyi. A fuller 

 description of the histology and histogenesis of the tissues will be given in the parts which treat 

 of the ditterent organs in detail. 



A profile view of the larva as it appears while still inclosed by the eggsiiell and of one imme- 

 diately after hatching is seen in PI. xxi. Figs. 1 and 5, and the brief and insignificant metamor- 

 phosis which is required to i>rovide it with the adult (!baracters are illustrated and describeil in 

 a separate paper (Pis. xxi-xxtv). 



Most noteworthy are the large, stalked, compound eyes, the segmented abdomen provided 

 with its full number of appendages, the short, stumpy antennic, and tlie swollen chelae or pincers 

 of the first pair of thoracic legs. At this stage this .\lplieus is a larva, but in a restricted sense, 

 since many adult characteristics are present. It is a larva, with i)reparations for immediately 

 assuming the adult state. Some of the larval peculiarities are the spatulate telson, the biranious or 

 schizopodal pereiopod.s (first to fourth pair, inclusive), the rudimentary pleopods, the unabsorV)ed 

 food yolk, and the uncovered, stalked eyes. 



.The structural details are now very great, .so that it is often impossible to interpret the parts 

 seen in a single section, and it is only by comparing sections made in different planes that the 

 relations of the organs can be successfully made out. 



In Fig. 19G (PI. Liii) the plane of section is nearly vertical and median throughout, except 

 for the posterior half of the abdomen. The supra-cesophageal ganglion, which is usually spoken of as 

 " the brain "(s.o.f/.), is a complex organ, composed of internal, jncAhiWury innssci^{pi( nl-txuh.stanz halln), 

 and cellular tissue which completely invests them. It is made up of the fused ganglia of at lease 

 two segments, those of the first and second antennte. This fusion is complete from the early stages 

 of development, and the relations of the parts are now extremely complex. They are best illus- 

 trated by a comi)arison of the series of transver.se sections (Pis. liv, I.V, Figs. 211-219) with 

 those made in a horizontiil plane (PI. LVii, Figs. 238-243), and it will be seen that there are four 

 pairs of fibrous masses in the brain, intimately connected together. 



These compact and finely granular masses in the interior of the ganglia of invertebrates were 

 described by Leydig twenty-five years ago under the name of Punkt.substnnz and later by Dietl 

 (187C) as Marksubstanz. As Krieger remarks, the latter name is bad, since it confuses this tissue 

 with the spinal marrow of vertebrates, with which it has nothing to do. It is essentially a/rlt of 

 very fine fibers. \V«^ will therefore speak of it as the Punktsiibsfnnz, or, to use a more descriptive 

 term, the Jibroiti subxtancc of the ganglia. 



The first i)air of these, the anterior or optic fihroux maiises (PI. LV, Figs. 212-21.5), are the 

 largest. They are completely fused on the middle line and form a single compa(!t mass, which is 

 slightly constricted laterally (PI, lvii, Fig. 242, of.) and which is divided in front (PI. liv, Fig.s. 

 210, 211), where it gives oft" two diverging stems of fibrous tissue (sometimes (tailed optio nerrcs) to 

 the optic gauglia in the stalks of the compound eyes (see also Pi. lvii, Fig. 210 of.). 



