MEMOIRS OP THE NATIONAL ACADEMY OF SCIENCES. 389 



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

 tion ? There are some facts which favor the view that it has done so, but before the question can 

 be definitely settled we must determine more precisely how far intermediate or aberrant forms 

 represent phases of the individual and of the race. It is not probable that we are here dealing 

 with the hybrids between two originally distinct species. 



PAET SECOND. 

 THE DEVELOPMENT OF ALPHETJS. 



I. STRUCTURE OF THE I.ARVA. 



(PI. XLIX, Fig. 174. PI. LIII, Fig. 196. Pis. LIV-LVII.) 



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

 early phases of segmentation and extend to the larval and adult periods. lu order that the prog- 

 ress of development may be followed in the light of the structure which the embryo finally attains, 

 we will start with a general survey of the anatomy of the first larva of Alphcust xaulcyi. A fuller 

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

 of the different organs in detail. 



A profile view of the larva as it appears while still inclosed by the eggshell 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 provide it with the adult characters are illustrated and described in 

 a separate paper (Pis. xxi-xxiv). 



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

 with its full number of appendages, the short, stumpy antennae, and the swollen chehe or pincers 

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

 since many adult characteristics are present. It is a larva, with preparations for immediately 

 assuming the adultstate. Some of the larval peculiarities are the spatnlate telson, the biramous or 

 schizopodal pereiopods (first to fourth pair, inclusive), the rudimentary pleopods, the unabsorbed 

 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. 196 (PI. LIII) the plane of section is nearly vertical and median throughout, except 

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

 "the brain "(s.o. #.), is a complex organ, composed of internal, medullary masses (punktsubstanz balls), 

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

 two segments, those of the first and second antenme. This fusion is complete from the early stagi-s 

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

 trated by a comparison of the series of transverse sections (Pis. LIV, LV, Figs. 211-219) with 

 those made in a horizontal plane (PI. LVII, Figs. 238-243), and itVill 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 Punktsubstanz ^uid later by Dietl 

 (1876) 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./Hf of 

 very fine fibers. We will therefore speak of it as the Pwnktsubstanz, or, to use a more descriptive 

 term, the fibrous substance of the ganglia. 



The first pair of these, the anterior or optic fibrous masses (PI. LV, Figs. 212-215), are the 

 largest. They are completely fused on the middle line and form a single compact mass, which is 

 slightly constricted laterally (PI. LVII, Fig. 242, of.) and which is divided in front (PI. LIV, Figs. 

 210, 211), where it gives off two diverging stems of fibrous tissue (sometimes called optionerres) to 

 the optic ganglia in the stalks of the compound eyes (see also PL LVII, Fig. 240 of.). 



