THE PROS OM A TIC SEGMENTS OF AMMOCCETES 287 



The Origin of the Suctorial Apparatus of Petromyzon. 



This powerful suctorial apparatus is innervated entirely by the 

 trigeminal nerve, so that here in its muscular arrangements any 

 original segmental arrangement of the muscles of mastication might 

 be expected to be visible, It consists of a large rod or piston, to 

 which are attached powerful longitudinal muscles ; a large muscle, 

 the basilar muscle, which assists the piston in producing a vacuum, 

 and annular muscles around the circular lip. 



Turn now to the full-grown larval form, Ammoccetes, an animal 

 in the case of Petromyzon Plancri as large as the full-grown Petro- 

 myzon, and seek for this musculature. There is, apparently, no sign 

 of it, no suctorial apparatus whatever, only, as already mentioned, 

 an oral chamber bounded by the lower and upper lips and the 

 remains of the septum between it and the respiratory chamber — the 

 velar folds. Attached to its walls a number of tentacles are situated, 

 which form a fringe around and within the mouth. Most extra- 

 ordinary is the contrast here between the larval and the adult 

 stages ; in the former, no sign of the suctorial apparatus, but simply 

 tentacles and velar folds ; in the latter, no sign of tentacles or of 

 velar folds, but a massive suctorial apparatus. 



In order, then, to understand the origin of the muscles of masti- 

 cation, it is necessary to study the changes which occur at trans- 

 formation, and thus to find out how the suctorial apparatus of the 

 adult arises. This most important investigation has been under- 

 taken by Miss Alcock, and owing to the kindness of Mr. Millington, 

 of Thetford, we have been able to obtain a better series in the trans- 

 formation process than has ever been obtained before. Miss Alcock 

 has not yet published her researches, but has allowed me to make 

 use of some of her facts. 



An enormous proliferation of muscular tissue takes place with 

 great rapidity during this transformation, which causes the disappear- 

 ance of the tentacles, and gives origin to the suctorial apparatus. 

 The starting point of this proliferation can be traced back in all 

 cases to little groups of embryonic tissue found below the epithelial 

 lining of the oral chamber in Ammoccetes. Of these groups the most 

 conspicuous one is situated at the base of the large median ventral 

 tentacles. Others are situated at the base of the tentacular ridge. 

 Further, although this extraordinary change takes place in the 



