306 THE ANATOMY OF IXVERTEBRATED ANIMALS. 



deed, but otherwise taking no share whatever in its changes. 

 The young Mysis might, therefore, in this condition be justly 

 termed a pupa, for the relation of the primitive integument 

 to the animal which it incloses is precisely that of the pupa 

 skin to the imago of an insect. 



The antennules and antennae remain intact within the 

 sheaths afforded by the primitive integument, but, becoming 

 immensely elongated and divided at their extremities, assume 

 more and more their proper adult conformation. 



In front of the antennules, a large rounded protuberance 

 makes its appearance upon each procephalic lobe, and event- 

 ually becomes the ophthalmic peduncle. At first, the sternal 

 portions of the somites, corresponding with these three pairs 

 of appendages, occupy the same plane with one another and 

 the posterior sterna (Fig. 79, F, G) ; but, by degrees, they 

 become bent up (Fig. 79, H)^ and at length the ophthalmic 

 sternum occupies the upper and front part of the head (Fig. 

 79, 7"). In this way the " cephalic flexure " is produced. 

 The mouth is indicated behind the antennary sternum, which 

 projects backward in the middle line to form the labrum. On 

 each side of it the rudiments of the mandibles appear, and 

 behind these are the papillary commencements of the two 

 pairs of maxillag. Behind the second pair of maxillas a dis- 

 tinct constriction indicates the commencement of the thorax, 

 the appendages of which appear, at first, as tubercular eleva- 

 tions, all of precisely the same character, and all directed 

 backward parallel with one another. The abdomen is at first 

 very small, and the appendages of its sixth somite early ac- 

 quire a far larger size than the others. The telson is devel- 

 oped from the middle line above the anus. While all these 

 changes are going on, the blastoderm gradually extends over 

 the tergal surface of the embryo and closes it in. When the 

 carapace is first distinguishable it appears as a ridge arising 

 from the sides of the posterior thoracic somites, beginning at 

 the last but one, and gradually extending forward as far as 

 the antennary somites. The ridge increases and becomes a 

 fold, which overhangs the bases of the thoracic appendages 

 (Fig. 79, G) ; and if this fold be turned back (Fig. 79, Z), its 

 actual attachments may be readily demonstrated. 



Having advanced thus far in its development, the foetal 

 Mysis, with all its organs fully formed, though somewhat 

 different in appearance from those of the adult, casts its pupa- 

 skin and straightens its body, which, from having its pos- 

 terior portion bent on the anterior, as in the embryo (Fig. 



