METAZOA COELENTERA. 123 



are single and are arranged four on each side of the 

 median line of the body they are called bilateral mesen- 

 teries. They can be distinguished from the biradial 

 mesenteries of the Hexactiniae s'nce these arise in pairs 

 that radiate from the center to the circumference. When 

 the animal is placed in accordance with this bilateral 

 arrangement, there is a dorsal and a ventral side. The 

 former differs from the latter by having the mesenteries 

 much shorter, as shown in fig. 26. These walls may be 

 seen on the outside in a three-and-a-half days' larva (fig. 

 27 ; fig. 28, the same contracted). 



It is at this early time or even a little earlier (seventy- 

 two hours) that buds appear, as will be described farther 

 on. 



Figures 29 and 30 show the larva when it has settled. 

 The tentacles are at first quite simple but during growth 

 become pinnate. The fact that, according to Wilson, the 

 eight mesenteries and eight tentacles appear at one time 

 and not in sequence, as is the case in the Hexactiniae, 

 does not prove that the Alcyonaria are more specialized 

 than the Hexactiniae. In order to demonstrate this 

 view, evidences of the reduced character of the Hexac- 

 tiniae, such as the reduction of biradial mesenteries, 

 must be brought forward. The abbreviated record in 

 the development of forms like Renilla tends to prove that 

 these are more specialized members of the Alcyonaria, 

 and we should predict that the primitive forms of the 

 group would show a regular sequence in the appearance 

 of the mesenteries. 



The spicules of Renilla are formed, according to 

 Wilson, in both the endoderm and the ectoderm. Those 

 of the endoderm appear first ; they are oval nodules and 

 are not numerous. Fig. 31 represents a young stage; fig. 

 32, an old stage ; and fig. 33, the spicule taken from the 

 cell. The spicules of the ectoderm appear soon after 

 the attachment of the larva. They are at first rod-like 

 and colorless, and not until a colony is formed, do they 



