Sea-Pir. 161 



they become cylindriform ; and finally they attach themselves by 

 one extremity which widens into a Hiydrorhiza/ and develope a 

 circlet of tentacles and a digestive sac at the other. In the family 

 Campanularidae^ which differs from the Sertularidae mainly in the 

 pedunculation of its cells, many species have Medusae set free from 

 their gonophores. The outer layer of the ectoderm has secreted 

 a firm but flexible polyj)ary, which is continued into the cups 

 for the lodgment of the digestive and generative zooids, as ' hydro- 

 thecae^ s. ^ calycles' s. 'cells/ and as 'gonothecae' s. 'capsules/ in 

 the language of different writers. The digestive or alimentary 

 zooids are known as 'polypites/ their cavity, which is not a 

 distinct sac freely suspended, but a mere hollow scooped out in 

 the part of the coenosarc which is prolonged into each cell, is 

 continuous with that of the coenosarc or ' coenenchyma' by a 

 narrow tubular passage, the ' transition piece'' of Reichert, passing 

 inwards from the bottom of the stomach. Representations of two 

 other sub-kingdoms may be seen to have afiixed themselves semi- 

 parasitically to the main stems of this zoophyte. One of them is 

 the Spirorbis, a small Tubicolar Annelid, with a discoidal shell, 

 somewhat like that of the fresh- water mollusc Planorbis ; the other 

 is one of the Cyclostomatous Polyzoa, Tubulipora patina, which with 

 its aggregated calcareous cells presents an appearance not unlike 

 that of a small tubiflorous flower belonging to a plant of the order 

 Compositae. 



Coelenterata, with an external polypary, and a uniserial circlet 

 of tentacles such as this specimen possesses, bear a superficial re- 

 semblance to many Polyzoa, which indeed were formerly classed 

 with tlaem. But beyond these more or less unimportant points of 

 resemblance, the Hydroid Polypes and the Polyzoa have scarcely 

 any points of real similarity, unless we reckon as such the absence 

 of ducts to the generative glands, and the power of multiplying by 

 gemmation, properties which attach to them however in common 

 with representatives of several other classes of Invertebrata. In 

 the absence of a digestive tube differentiated from the peri-visceral 

 cavity; in the absence of muscles differentiated into distinct fas- 

 cicles and crossing that cavity ; in the absence of any nerve-centre, 

 and in the presence of thread-cells (all points to be made out, and, 

 with the exception of the one relating to the nerve system, with 

 very little trouble, under the microscope, with a fresh specimen of the 



M 



