430 , MEMOIRS OF THE AMERICAN ACADEMY. 



the adult to leave no doubt of its identity with the Balanoglossus raised from 

 Tornaria. The differences between them are simply differences of size and quantity. 

 The collar is more distinct, the number of gills greater, and the convolutions of 



more 



vibratile 



numerous ; the anal extremity has lost completely its circle of 

 The walls of the body are scarcely more opaque than in the young 

 Balanoglossus raised directly from Tornaria, with the exception of the proboscis, 

 in which the muscular bands, both transverse and longitudinal, are more fully de- 

 veloped. The water-system and the heart could only be seen by compression ; the 

 latter, closely contracted, appeared like a solid ball at the base of the proboscis 

 within its cavity. The anterior opening of the cavity of the proboscis is very 

 distinct : the posterior opening is situated on the ventral side, immediately in advance 

 of the mouth ; it is quite a long, slender slit. The so-called skeleton of the proboscis 

 is very different from that figured by Metschnikoff and Kowalevsky ; it is pointed 

 anteriorly, with two branches curving towards the ventral side, pointing towards the 

 posterior base of the proboscis. It consists of two parts, — one apparently chitine, 

 semi-transparent, forming the two bent forks arching towards the mouth and the 

 other the main shaft from which the forks arise. At the head of the shaft there is a 

 flat saucer-shaped expansion, of granular character, quite flexible, a sort of support for 

 the base of the limber proboscis. From the base of the proboscidal skeleton there 

 extends a granular chord as far as the extremity of the gills, to the sides of which 

 the three branched, semi-transparent supports of the gill-folds are attached; this 



an internal skeleton in the anterior part of Balanoglossus, supporting the 

 gill-folds, which is without parallel in the Invertebrates, unless we except some of 

 the Ascidians. The nature and function of this skeleton is not known ; the portion 

 at the base of the proboscis may be homologized to the operculum of some of the 

 Annelids, with which I am inclined to associate Balanoglossus. The gills are numerous, 

 in many pairs, along the dorsal line of the anterior part, but as yet forming only 

 slightly complicated folds, not to be compared with the complicated folds of the gills 

 of the adult; and thus the resemblance to the young Balanoglossus raised from 

 Tornaria is still quite complete. At the terminal extremity of the gill-system we 

 find young gills developing from the oesophagus exactly as we have seen them 

 develop in Tornaria. 



With this preface I can now describe our common species, which differs in some 

 very important points from the two species of Balanoglossus distinguished by Kowa- 

 levsky. I shall follow his nomenclature, and as I have nothing of any great importance 

 to add to his exhaustive anatomy of the adult, will only compare our species with 



foi 



