THE DEVELOPMENT OF ASTACUS. 291 



of the abdomen, and that there is no telson. This is devel- 

 oped only at a much later period from the dorsum of the end 

 of the abdomen, and, by its outgrowth, forces the anus to 

 the ventral side of the body. 



In the mean while, the oral aperture is developed behind 

 the labrum, which moves backward ; while the maxilhe, max- 

 illipedes, and ambulatory feet appear in succession as eleva- 

 tions or ridges of the substance of the embryo, which are, at 

 first, all alike, and gradually become specialized into their 

 ultimate forms. 



When these appendages first appear, the maxillse and first 

 pair of maxillipedes are attached to the embryo in front of 

 the thoracico-abdominal process, the second maxillipedes lie 

 in the angle between them, and the third maxillipedes and 

 following appendages are attached to the sternal surface of 

 the thoracico-abdominal process itself; and, as this process is 

 at first bent forward upon the rest of the germ, it follows that 

 the appendages attached to it look upward, while those at- 

 tached to the anterior part of the embryo look downward. 

 As development proceeds, however, the embryo gradually 

 straightens itself, more and more of the anterior part of the 

 thoracico-abdominal process becoming continuous in direction 

 with the anterior part of the embryo ; until, at length, the 

 whole of the cephalo-thoracic portion forms a convex surface, 

 parallel with the vitellary membrane, only the abdomem re- 

 maining bent upon the cephalo-thorax. The middle portion 

 of the carapace is formed by the continuous calcification of 

 the dorsal walls of the cephalo-thorax of the embryo. Its 

 pleura are developed as two distinct folds, one of which, the 

 rudiment of the branchiostegite, encircles the embryo poste- 

 riorly, and extends forward on each side as far as the mandi- 

 bles ; while the other, the rudiment of the rostrum, and an- 

 terior cephalic pleura, is developed in front of the eyes, and 

 extends on each side to meet the former. Rathke's clear ac- 

 count of this matter is in perfect accordance with what I have 

 observed in 3ft/sis, and shows conclusively that the carapace 

 is not developed from any one or two somites in particular, 

 but that its tergal portion corresponds with, and is formed by, 

 the terga of all the cephalo-thoracic somites, while the bran- 

 chiostegites and rostrum are developments of the lateral por- 

 tions of all these somites; in fact, represent their pleura, 

 which, like the terga, are connate and continuously calcified. 



The appendages are thus, at first, similar to one another, 

 and each consists of a ridge which eventually takes the form 



