1 10 Introduction to Animal Morphology. 



section an actinozoon appears as two concentric tubes, 



whose walls are joined by the radial mesenteries, while a 



hydrozoon appears as a single Fig. 16. 



tube. Striped muscle, several 



forms of connective and gland 



tissue exist in some ; but they 



have no nervous nor vascular 



system. The ectoderm consists 



of two laminae, an outer, ecderon 



or epidermis, of epithelium, 



pigment, clear irregular gland 



Cells, and trichoCyStS With be- Section of Sea Anemone ;i. month; 

 i 1 -i r--\ r m 'i primary mesentery ; 



bristled filaments, growing from <i.u- y m ,. s ,.iit-ry : *. ectoden 



. dooern ; f, 



within outwards; and an inn. . 



enderon, growing from without inwards, made of 

 granular, striated connective tissue. The endodcrm 

 consists of a muscular layer of circular and longitu- 

 dinal fibres (the former strongly developed in the 

 tentacles and around the mouth ; this stratum may be 

 intermediate or ectodermal in origin), a connective 

 lamina, and an internal ciliated epithelium. The 

 tentacles are hollow, contractile, or fully retractile, 

 lined by endoderm, and both sensitive and prehensile ; 

 Ix -tween these and the mouth is a peristomial space. 

 The mesenteries are arranged in several orders suc- 

 cessively developed : primary, reaching from the body 

 to the stomach wall ; secondary, not extending to the 

 stomach, and placed between the primaries, &c. 



A skeleton may be formed in the ectoderm by the in- 

 clusion of sponge spicules and other foreign bodies (Sagnrtia 

 .Schilleriana, Heterozoanthus), but usually by the deposits of 

 calcium carbonate in the connective laminae. These deposits 

 may be (i) discontinuous, of small, definitely shaped, spicular 

 bodies, as in Alcyonidae ; or (2) continuous, coherent skele- 



