122 SYNOPTIC COLLECTION. 



of motion, but afterwards become active, propelling the 

 larva through the water. When the larva is twenty- 

 eight hours old the ends of the ectoderm cells are swollen 

 and bulbous, as seen in fig. 21, and the ball-like portion 

 separates off to form the supporting lamella or mesoderm 

 (fig. 22, m). At the end of forty-eight hours all the 

 central cells not used in the formation of the endoderm 

 have been absorbed as food by the endoderm cells and 

 the central region is an empty cavity. In this case, as 

 pointed out by Wilson, digestion in the young Renilla is 

 intra-cellular or amoeboid. This is the most primitive 

 mode of digestion and is probably inherited by the Pori- 

 fera and Coelentera from the Protozoa. 



Usually between the fortieth and fiftieth hour the 

 internal bag is formed and the central cavity is divided 

 into chambers by mesenteries. The cells at the large 

 end of the body increase rapidly and are pushed inward 

 in a solid mass (fig. 23,^), forming a plug. In time a 

 narrow cavity is seen in its center (fig. 24, c), This 

 cavity has no communication with the exterior for twenty 

 to twenty-five hours. At the end of this time the cavity 

 breaks through and a mouth is formed ; afterward an 

 opening occurs at the lower end, placing the internal bag 

 in communication with the central cavity. As already 

 stated, the mesenteries appear at the same time that the 

 bag is formed. They consist of eight thick plates- of 

 endoderm cells which extend down from the oral end of 

 the body and are of different lengths. These are seen 

 in fig. 25, which is a cross section of the anterior part of 

 a forty-eight hours' larva, and in fig. 26, a cross section 

 of the posterior portion of a four days' larva. In the 

 living animal thes.e mesenteries arise as bilateral organs ; 

 that is, they appear one on either side of the median 

 plane of the body. (The figures 25 and 26 should be 

 turned slightly to show the bilateral condition more 

 plainly. The dotted line in fig. 26 indicates the median 

 plane of the body.) For the reason that these partitions 



