Manchester Memoirs, Vol. /xii, (191 7) No. 4 5 



siphonozooids are exhalent as well as inhalent in function is proved 

 for Pennatula rubra by the observation of Mrs. Musgrave (Q.J.M. 

 S. 1909, p. 455) that "clouds of methylene blue squirted among 

 them were immediately dispersed in an outward direction. ' ' The 

 fact that P. rubra has mesenteric filaments appears to confirm the 

 theory that these are exhalent in function. 



It must further be noted that in a few species of Pennatulacea 

 there occur large zooids recently named Mesozooids by Professor 

 Hickson (1916, p. 11) which are apparently exhalent in function. 

 These mesozooids are characterised by having no tentacles, and "a 

 large open stomodaeum with a weak siphonoglyph supported by 

 eight mesenteries provided with strong muscle bands," hence the 

 water is probably ejected by the forcible contraction of the zooids 

 by means of these muscles. They are found in Pennatula Murrayi, 

 P. grandis, and many species of Pteroeides. It is suggested by 

 Hickson that the exhalent zooid of Renilla (Wilson 1883) may be 

 of the same nature, also the "Scheitelzooiden" described by 

 Jungerson (1888) at the distal end of the rachis of young 

 Pennatula phosphor ea colonies. Zooids resembling mesozooids 

 occur on the ventral side of the rachis in Sarcophyllum. 



Conclusions. 



In view of the foregoing facts it appears probable that all the 

 more primitive species are provided with dorsal mesenteric fila- 

 ments which subserve the function of causing exhalent currents, 

 and in the one genus of these families where filaments are lacking, 

 namely Renilla, an exhalent zooid is present. It is possible that 

 Renilla should be considered to be a specialised and not a primitive 

 form, the exhalent zooid marking a certain degree of specialisation. 



In the higher genera the mesenteric filaments are supplanted by 

 zooids specially modified for the function of exhalence. This com- 

 clusion is tenable for the cases of P. Murrayi, Sarcophyllum and 

 most of the species of Pteroeides where filaments are absent 

 and mesozooids present. In other species of Pennatula, P. 

 phosphorea P. rubra, P. fimbriata, where mesozooids are not 

 developed, the dorsal filaments do occur. 



In Pennat. grandis, Pter. pellucidum, Pter. Steenstrupii, meso- 

 zooids and mesenteric filaments in the siphonozooids are present 

 together. As these species closely resemble others of their genera 

 in all essential characters, it can scarcely be held that the persist- 

 ence of filaments denotes a lower grade of development, or that 

 with further evolution the filaments would disappear and the 

 specialised mesozooids acquire the exclusive function of exhaling 

 water. As far as Pter. Steenstrupii is concerned the presence of 



