DEVELOPMENT OF WORMS 949 



aeration of both fluids ; for the first is transmitted to the teiidril- 

 like tentacles which surround the mouth (fig. 715, >, 6), whilst the 

 second circulates through the beautiful arborescent gill-tufts (k, k) 

 situated just behind the head. The former are covered with cilia, the 

 action of \vhich continually renews the stratum of water in contact 

 with them, w r hilst the latter are destitute of these organs ; and this 

 seems to be the general fact as to the several appendages to which 

 these two fluids are respectively sent for aeration, the nature of their 

 distribution varying greatly in the different members of the class. In 

 the observation of the beautiful spectacle presented by the respiratory 

 circulation of the various kinds of Annulates which swarm on most 

 of our shores, and in the examination of what is going on in the 

 interior of their bodies (where this is rendered possible by their 

 transparence), the microscopist will find a most fertile source of 

 interesting occupation ; and he may easily, with care and patience, 

 make many valuable additions to our present stock of knowledge on 

 these points. There are many of these marine worms in which 

 the appendages of various kinds put forth from the sides of their 

 bodies furnish very beautiful microscopic objects ; as do also the 

 different forms of teeth, jaws, &c. with which the mouth is com- 

 monly armed in the free or non-tubicolar species, which are 

 eminently carnivorous. 



The early history of their development is extremely curious ; 

 for many come forth from the egg in a condition very little 

 more advanced than the ciliated gemmules of polypes, consist- 

 ing of a globular mass of untransformed cells, certain parts of 

 whose surface are covered with cilia, which ordinarily become 

 arranged in one or more definite rings ; in a few hours, however, 

 this embryonic mass elongates, and the indications of a segmental 

 division become apparent, the head being (as it were) marked off 

 in front, whilst behind this is a large segment thickly covered with 

 cilia, then a narrower and non-ciliated segment, and lastly the 

 caudal or tail segment, which is furnished with cilia. A little 

 later a new segment is seen to be interposed in front of the 

 caudal, and the dark internal granular mass shapes itself into the 

 outline of an alimentary canal. 1 The number of segments pro 

 gressively increases by the interposition of new ones between the 

 caudal and its preceding segments; the various internal organs 

 become more and more distinct, eye-spots make their appearance, 

 little bristly appendages are put forth from the segments, and 

 the animal gradually assumes the likeness of its parent ; a few 

 days being passed by the tubicolar kinds, however, in the actively 



1 A most curious transformation once occurred within the Author's experience 

 in the larva of an Annelid, which was furnished with a broad collar' or disc fringed 

 with very long cilia, and showed merely an appearance of segmentation in its hinder 

 part ; for in the course of a few minutes, during which it was not under observation, 

 this larva assumed the ordinary form of a marine worm three or four times its pre- 

 vious length, and the ciliated disc entirely disappeared. An accident unfortunately 

 prevented the more minute examination of this worm, which the Author would have 

 otherwise made ; but he may state that he is certain that there was no fallacy as to 

 the fact above stated, this larva having been placed by itself in a cell, on purpose 

 that it might be carefully studied, and having been only laid aside for a short time 

 whilst other selections were being made from the same gathering of the tow-net. 



