180 



COELENTBRATA. 



upon it in different ways. In Veretillum they are distributed all round the 

 rachis ; in Kophobelemnon (Fig. 147) they are absent on a streak of one side 

 the so-called ventral side ; in most genera they are arranged bilaterally, there 

 being a dorsal as well as a ventral streak free from them. Further, in some 

 genera they are sessile on the rachis, but in the Pennatulea, or Sea-Feathers 

 proper, the polyps are borne only on lateral processes of the coenenchyma, called 

 the pinnules. The pinnules are broad triangular leaf-shaped structures attached 

 by their base to the rachis and carrying the polyps on their dorsal edges (Fig. 

 148). The polyp-tubes project in fome cases to form the so-called cells; the 

 cells may have spines or tufts of spicules. 



The polyps are dimorphic ; the autozooids have tentacles and generative organs, 

 and are without a gonidial groove ; the siphonozooids possess a gonidial groove 

 but are without tentacles and gonads, also they have filaments on the two dorsal 



mesenteries only. The siphon o- 

 zooids are distributed over the 

 whole rachis in Renilla and Vere- 

 tillum ; and in the Pennatulea 

 they are on the rachis or on the 

 pinnules. 



The stalk generally contains an 

 axial calcareous or horny rod sur- 

 rounded by a sheath of epithelial 

 cells ; and the coenenchyma and 

 bodies of the polyps may contain 



FIG. 148. Pennatula Sulcata Roll, (after Kolliker). 

 A, from the dorsal ; B, from the ventral side. 



isolated spicules. The polyps are 

 continued into tubes which join 

 the canal system of the coenen- 

 chyma. This canal-system con- 

 sists of large canals continued 

 down from the polyps and open- 

 ing after a longer or shorter 

 course into a few "main canals," 

 which run in the stalk. There 

 are generally four of the latter 

 (two in Renilla}, of which two 

 are lateral, one dorsal, and one 

 ventral. At the lower end of 



the stalk the lateral canals cease, leaving only the dorsal and ventral. They 

 fuse at the end of the stalk and are said to open there. In addition to the 

 large canals there are minute canals uniting them. Some of the polyps are 

 closed below, and open at their base by narrow openings into the general canal 

 system of the colony. 



The Pennatulacea are not distributed uniformly over all seas. They are 

 mainly littoral, but deep water forms are known, and these, it is important to 

 notice, belong principally to the simpler families, e.g., the Protoptilidae and 

 Umbellulidae. They appear to be absent, or nearly so, in the deeper parts of 

 the Pacific and Atlantic Oceans and the South Polar Sea at a certain distance 

 from the shore. 



Section 1. Pennatulea. 



The Sea-feathers. With pinnules ; rachis with a bilateral arrangement of the 

 polyps, elongated, cylindrical. 



