1887.] Balanoglossus Larva from the Bahamas. 



149 



groove is seen to have disappeared altogether, while that behind the 

 proboscis has become much shallower. The external appearance is 

 complicated by the formation, on each side of the proboscis, of a 

 fH-shaped groove, the middle limb of which communicates with the 

 post-proboscidean groove, while the margins of all the limbs are pro- 

 vided with short, broad tentacles. The arrangement of this groove is 

 indicated by the shading between dotted lines in fig. 3. 



Sections show that the mesoblastic organs have undergone con- 

 siderable reduction, both relative and, in some cases, absolute. The 

 proboscis cavity is smaller (fig. 3, I) ; its walls are thinner, and its 

 muscles fewer. The notochord beneath it has quite disappeared ; 

 the collar cavities have disappeared ; while the trunk cavities are 

 small and thin-walled (fig. 3, III). No trace of gill-pouches remains. 

 The ectoderm is much thinner, and contains hardly a trace of any 

 nervous structure except the much diminished " Scheitelplatte " 

 (Sch.), on which the eye-spots still persist. 



In connexion with this degeneration of the tissues, it may be 

 noticed that many cells, possibly phagocytes, are present in the 

 blastoccsl at earlier stages (fig 2, Ph. ?) . 



I was not able during my stay at Bernini to follow this creature 

 further ; but at Nassau, New Providence, in the middle of the Bahama 

 bank, I observed, during four months, a similar series of changes in a 

 much larger larva. This larva was first obtained at a period just 

 before the development of the tentacular apparatus, and after the 

 disappearance of the collar groove (if this ever existed). The collar 

 and trunk cavities were both well developed, and the proboscis cavity, 

 with its gland and pore, was as in the youngest Bernini forms. Eye- 

 spots were present, and there was a well-developed cutaneous nerve 

 plexus. In this form degradation was followed to a much fuller 

 extent, till the ectoderm was (except on the well-developed tentacles 

 and beneath the cilia) a mere flattened epithelium ; the trunk cavity 

 was a minute solid rod beneath the ciliated ring ; the collar cavity 

 disappeared, and the reduction of the proboscis cavity was carried 

 much further than in the Bernini form. 



I hope to publish a fuller account of both forms in a subsequent 

 paper. In the meantime, it is submitted that there is fair ground for 

 the belief that the organisms described are Balanoglossus larva?, 

 which from some cause or other have been unable to develop adult 

 characters, and have therefore varied. Independent evidence shows 

 that a probable cause may be the compulsory shifting of the larva? 

 into deep water by the joint action of currents and winds.* 



If this be admitted, four things follow : — First, that, at least in 

 some cases, the transmission by a larva of hereditary changes is only 



* These larvee were practically all caught outside the 100-fathom line. 



