I lo Introdudtoji 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. i6. 



tube. Striped muscle, several 



forms of connective and gland 



tissue exist in some ; but they 



Tiave 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 ofSea Anemone ;«/, mouth; 



. , _ ~, . P /«', primar)' mesentery ; wj", second- 



briStlednlamentS, grOWmgirOm dary mesentery; e, ectoderm ; e, en- 



doderm ; /, tentacle ; g; generative 

 Wlthm outwards ; and an inner, element ; J, disc ; s, body cavity. 



•enderon, growing from without inwards, made of 

 granular, striated connective tissue. The endoderm 

 ■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 ; 

 between 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 (Sagartia 

 -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- 



