36 THE SEA-ANEMONE 



(b) The gullet or stomodseum. This has the form of a flattened cylinder 



hanging down into the interior of the body. It is marked with longi- 

 tudinal grooves, two of which, termed the ciliated grooves or siphono- 

 glyphs, occur at the ends of the long diameter of the gullet, and 

 correspond i;o the corners of the mouth. They are broad and deep and 

 sometimes bear cilia. The gullet is composed of ectoderm, mesogloea 

 and endoderm. 



(c) The mesenteries. These consist of supporting mesogloea, covered on 



both sides with endoderm. Muscles occur on the mesenteries which 

 are divisible into the following groups : — 



(i) Primary mesenteries, usually twelve in number, all reaching to 

 the gullet. They are arranged in six pairs. The cavity en- 

 closed by each pair is known as the intraseptal cavity ; the 

 cavity between adjacent pairs is the interseptal cavity. The 

 two pairs attached to the siphonoglyphs of the gullet divide 

 the animal into bilaterally symmetrical halves and are called 

 the directive mesenteries. The retractor or longitudinal muscles 

 on these directives are turned away from each other, and 

 are within the interseptal spaces ; while on all the other 

 mesenteries the muscles are facing, and within the intraseptal 

 spaces. 



(ii) Secondary and tertiary mesenteries arise later in the inter- 

 septal chambers and are short, not reaching to the gullet. 

 In all these the longitudinal muscles of each pair face each 

 other. 



The gonads or reproductive organs are developed in the substance of the 

 mesenteries, and are derived from the endoderm. 



Longitudinal Section. 



In a longitudinal section the following features may be seen : — 



(a) The body- wall, consisting of ectoderm, mesogloea, and endoderm. 



(b) The basal disc, similar to the body-wall, by which the animal fixes 



itself. 



(c) The gullet, with the longitudinal ridges. This hangs down into the body 



and terminates in a free edge produced at each end of the long diameter 

 into a descending lobe or lappet. 



(d) A primary mesentery. Its upper half is attached to the gullet, its lower 



half is free. It is attached to the basal disc and to the body-wall. In 

 development it arises as a growth from the body-wall to the gullet. 

 An aperture, the ostium, occurs in the mesentery and places the 

 adjacent compartments of the enteron in communication. 



(e) A secondary and a tertiary mesentery. These do not reach the gullet. 



their edges are free in the enteric cavity. 

 (/) The mesenterial filaments on the free edges of the mesenteries. Gland 

 cells, which secrete a digestive ferment, are present in the epithelium 

 of these filaments. 



