628 ANNELIDA! — CIRCULATION; DEVELOPMENT. 



(k, h) situated just behind the head. The former are covered 

 with Cilia, the action of which continually renews the stratum 

 of water in contact with them, whilst 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. The red fluid 

 is commonly considered as Blood, and the tubes through which 

 it circulates as Blood-vessels ; but the Author has elsewhere 

 given his reasons * for coinciding in the opinion of Mr. Huxley, 

 that the Colourless Corpusculated fluid which moves in the Peri- 

 visceral cavity of the body and in its extensions, is that which 

 really represents the Blood of other Articulated Animals ; and 

 that the system of vessels carrying the red fluid is to be likened 

 on the one hand to the 'Water- vascular system' of the inferior 

 Worms, and on the other to the Tracheal apparatus of Insects 

 (§524). — In the observation of the beautiful spectacle presented 

 by the Respiratory Circulation of the various kinds of Annelids 

 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 ren- 

 dered 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 Annelids, 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 commonly armed in the free or Non- 

 Tubicolar species, these being eminently Carnivorous. 



489. The early history of the Development of Annelids, too, is 

 extremely curious ; for they come forth from the egg in a condition 

 very little more advanced than the Ciliated Gemmules of Polypes, 

 consisting of a globular mass of untransformed cells, certain parts 

 of whose surface are covered with cilia ; in a few hours, however, 

 this Embryonic mass elongates, and 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, f The number of segments pro- 



* See his "Principles of Comparative Physiology," 4th Edit., 

 §§ 218, 219, 292. 



t A most curious transformation once occurred within the Author's 

 experience in the Larva of an Annelide, which was furnished with a 

 broad collar or Disk fringed with very long Cilia, and showed merely an 

 .appearance of segmentation in its hinder part ; for in the course of a few 

 /jiinutes, during which it was not under observation, this larva assumed 



