IV. CTENOPHORA. 261 



(fig. 208) and Sclerophylla (fig. 207) are solitary. Others, like Oculina,* 

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

 the only true coral in New England; Astrcea, the brain corals (Cceloria, 

 fig. 215, Diploria, Manicina); Cladocora (fig. 216), Favia (fig. 217). 



FIG 218. Madrepora erythrcea. (After Klunzinger.) 



(2) FUNGI ACEA, 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) POROSA, 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 medusae, 

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

 strong current tears them, and no attempts to preserve them have 

 been successful. The body is almost always biradially symmetrical; 

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

 metrical halves. Since the longitudinal axis is usually longer than 

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

 pear-shaped. In Cesium 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 composed of a soft jelly with con- 

 nective-tissue cells, penetrated in every direction by polynucleate 

 muscle cells branched at their ends and apparently 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. 22 IB, p) at the aboral 

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

 considerable resemblance to an otocyst (fig. 222). The thick 

 sensory epithelium 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 



