The mesenterial Filaments of the Alcyonaria. 17 



why it is that the zooids of some Pennatulida do uot possess these 

 Organs. The defective state of our anatomical knowledge will not 

 admit of a complete explanation, but in most cases it appears tolerably 

 clear. In Alcyotiium the cause appears to lie in the eircumstance that 

 the canal-system commuuicates on all sides with the bud, 

 which moreover lies at the same level with the canals. 

 We see in fig. 3 that the dorsal filaments, d.f., are still rudimentary, 

 though the entodermie filaments are well-developed. If we compare 

 this figure with fig. 1 3 [Funiculina) we see that the relation of the bud 

 to the canal-system is entirely different in the two cases ; so that, while 

 in Funiculina the upward currents are clearly advantageous , in Al- 

 cyonium they could be of no use. Hence the rudimentary state of the 

 dorsal filaments in the latter. If, now, we examine a later stage of 

 Alcyonitim (fig. 4) we find that the dorsal filaments, d.f., have far 

 outstripped the others. We find also that the bud has now a 

 different relation to the canal-system. The canals still com- 

 municate on every side with the bud, but the latter has elongated in 

 such a way that the Oesophagus and growing parts are situated quite 

 at the upper end. The principal nutritive supply must hence 

 come from below, and the dorsal filaments have therefore 

 elongated to produce the necessary upward currents. 



If the Position of the dorsal filaments be examined it is found to bc 

 the most advantageous possible. The canal-system comumnicates with 

 the polyp uot only along the sides but also along the middle dorsal 

 line through the round openings represented in the figure bctween the 

 dorsal filaments. Hence each filameut lies between two rows of open- 

 ings, and the advautage of this arrangement is obvious. 



We may now consider two other exceptions namely Paralcyonium 

 and Gorgonia. Here, as in Alcyonium^ the dorsal filaments appear at 

 the same lime with the others, and only in later stages become distinctly 

 longer. The case seems a difficult one at first, because bere as in Fu- 

 nictdina^ the communication with the canal-system is at the base of the 

 bud. But the explanation is clear, though different from that of the 

 Alcyoniuni-([Q,WQ\o^mtni. The canal-system oi Paralcyonium, as pointed 

 out by KocH , is reduced to a to nothing more than a series of large 

 holes which open directly into the cavity of the older polyp. The cur- 

 rents of the latter can therefore flow directly into the bud, as I have 

 observed in living specimens. In fact, the bud is only separated from 

 the main polyp by two or three delicate Strands of tissue which can 

 scarcely afford a resistance to the passage of the nutritive fluids. More- 



MittheilungGU a. d. Zoolog. Station zu Neapel. Bd. V. 2 



