xiii ALCYONARIA MESENTERIC FILAMENTS 3 33 



cases, are called the " autozooids," by the absence of tentacles, by 

 the absence of the six ventral and lateral mesenteric filaments, and 

 by the incomplete development of the muscles on the mesenteries, 

 and of the mesenteries themselves. They are, moreover, frequently 

 distinguished by the greater development and extent of the 

 ciliated groove or siphonoglyph on the ventral side of the 

 stomodaeum. 



It is often difficult to distinguish between true siphonozooids 

 and young autozooids, and consequently dimorphism has been 

 attributed to some genera in which it almost certainly does not 

 occur. Simple dimorphism undoubtedly occurs in the genera 

 Heteroxenia, Sarcophytum, Anthomastus, Lohophyium, Acrophytum, 

 and Pwragorgia. It has also been said to occur in Corallium 

 (Moseley and Kishinouye), Melitodes (Kidley), and some species 

 of Dasygorgiidae. 



The Pennatulacea are trimorphic. The main shaft of these 

 colonies is the much modified first formed or axial zooid, adapted 

 for the support of all the other zooids. It usually exhibits 

 no mouth, no tentacles, and only four of the original eight 

 mesenteries. It has no mesenteric filaments and no stomo- 

 daeum, and bears no sexual cells. The other zooids of the colony 

 are similar in structure to the autozooids and siphonozooids of 

 the dimorphic Alcyonaria. 



There are eight mesenteric filaments in all Alcyonarian 

 zooids. They have the appearance of thickenings of the free 

 edges of the mesenteries. Two of them, called the " dorsal " 

 mesenteric filaments, are straight when the anthocodia is ex- 

 panded, and extend from the edge of the stomodaeum for a long 

 distance down into the coelenteron of the zooid ; the other six, 

 called the "ventral" mesenteric filaments {i.e. the ventral and 

 ventro-lateral and dorso-lateral), are usually short and are almost 

 invariably slightly convoluted. The dorsal filaments are built 

 up of columnar cells provided with long cilia, and have usually 

 no gland cells, the others may show a few cilia but are principally 

 composed of non-ciliated gland cells. When the bolus of food 

 has passed through the stomodaeum it is seized by these ventral 

 filaments and rapidly disintegrated by the secretion of its cells. 

 The function of the dorsal mesenteric filaments is mainly 

 respiratory. During life their cilia produce a current which 

 flows towards the stomodaeum. On the ventral side of the 



