477 



veloped polypides. The sac is invaginated at its anterior 

 end. All that portion of it which lies behind the line of inva- 

 gination is thick and very irritable, and densely covered with 

 long cilia. From the invaginated portion of the sac the 

 double set of viscera were suspended as in the adult animal, 

 and were capable of partial exsertion, the invaginated por- 

 tion then rolling outwards upon itself to within a certain dis- 

 tance of the line of invagination, where its further evolution 

 was checked by bands in every way similar to the inferior 

 parieto- vaginal muscles of the adult. The two sets of viscera 

 were unequally developed, but in both the general structure 

 of the adult could be seen. The pharynx, stomach, and in- 

 testine could be traced, and the tentacular crown was also very 

 evident. The tentacula, however, were short and thick, and 

 they seemed less numerous and distinct than in the fully de- 

 veloped animal. Both the superior and inferior sets of parieto- 

 vaginal muscles were already very evident, but I could not 

 detect any others, though from the existence of a power of ex- 

 sertion and retraction it is pretty certain that at least the long 

 retractor muscles must have been present. The internal sur- 

 face of the common sac and the external surface of the sto- 

 mach were covered with a loose granular layer. From the 

 description now given of this little larva it will be seen that 

 the present account differs considerably from that given by 

 Meyen in the Isis, 1848. Meyen was the first to describe the 

 locomotive larvae of Alcyonella, but he mistakes the ciliated 

 sac for the external envelope of an egg containing two em- 

 bryos. This egg, he tells us, becomes ruptured at the anterior 

 extremity, and allows the embryos gradually to escape. The 

 bodies, however, described by Meyen and myself are physio- 

 logically of a nature totally different from eggs; they are in 

 reality embryos containing a double system of digestive and 

 respiratory organs, and destined to undergo an ulterior deve- 

 lopment in all their parts." 



The existence of egg-capsules with a circular aperture in 



