17. CTENOPEORA. 261 



(fig. 208) and Sderoxjhylla (fig. 207) are solitary. Others, like Oculma* 

 branch, and still others form compact masses. Astrangia danae (fig. 214), 

 the only true coral in New England; AstroRa, the brain corals {Coeloria, 

 fig. 21.5, Diploria, Manicina); Cladocora (fig. 216), Favia (fig. 217). 



Fig. 218. — Madrepora erythr<Ea. (After Klunzinger.) 



(2) Fdngiacea, or mushroom corals, with no outer wall to the coral. Some 

 are colonial, others (Fungia) are solitary. A sort of strobilation in de- 

 velopment. (3) PoEOSA, with skeleton porous like a fine sponge. Madre- 

 pora* deer's-horn coral (fig. 218), Porites, Astroides. 



Class IV. Ctenophora. 



The Ctenophores excel all marine animals, even the medusa?, 

 in transijarency and delicacy of tissues; many are so soft that a 

 strong current tears them, and no attempts to preserve t]}em have 

 been successful. The body is almost always biradially symmetrical ; 

 i.e., is divided by both sagittal and transverse planes into svm- 

 metrical halves. Since the loiigitndinal axis is usually longer than 

 the others, which are generally equal, the body is usually oval or 

 pear-shaj)ed. In Cestuiii the sagittal axis is greatly longer, giving 

 the animal the form of a band, whence the name ' Venus girdle." 



The bulk of the animal is com})osed of a soft jelly with con- 

 nective-tissue cells, jienetrated in every direction by polynucleate 

 muscle cells branched at their ends and apjDarently innervated by 

 special nerve cells. On the outer surface is a layer of ectoderm, 

 while in the interior is a system of branched entodermal canals. 



At the bottom of a depression (fig. 231B, p) at the aboral 

 pole is a thickened patch of ectoderm, the sense body, which has 

 considerable resemblance to an otocyst (fig. 2"2-2). The thick 

 sensory ei)ithelium forms a shallow groove, strong hairs which rise 

 from the edge of the groove arch over it, enclosing a space to be 

 compared to an incomplete vesicle. In the centre is a spherical 



