Invertebrata. Zoological Collection, 175 



and in sefven Macrourous genera, viz. Pagurus, Porcellana, Galatkea, 

 Crangon, Palcemon, Homarus, and Astacus. " These embrace all our 

 most familiar native genera of the Decapoda." The lobster, or Astacus 

 marinus, Mr Thompson states, " does actually undergo a metamorphosis, 

 but less in degree than in any other of the above enumerated genera, and 

 consisting in a change from a cheliferous Schizopode to a Decapode ; in its 

 first stage being what I would call a modified Zoecy with a frontal spine, 

 spatulate tail, and wanting the subabdominal fins ; in short, such an animal 

 as would never be considered what it really is, was it not obtained by 

 hatching the spawn of the lobster." In the other indigenous species of 

 AstacuSf Ast. jluviatilis, the river crayfish, it would appear, from the excel- 

 lent treatise of M. Rathke on the development of its eggs, that the young 

 are hatched in a form according with that of the fully grown animal. Mr 

 Thompson, however, suspects that some source of error may exist in these 

 observations. " If it should be found otherwise, it can oTily be regarded as 

 one solitary exception to the generality of metamorphoses, and will render 

 it necessary to consider these two animals for the future as the types of two 

 distinct genera." In illustration of the change of form observed by him in 

 the limbs of the lobster, Mr Thompson enclosed a sketch of the " chelife- 

 rous member of its larva," which is represented as divided to its base, and 

 consisting of, 1st, a cheliferous portion ; 2d, a portion of equal length with 

 the preceding, and terminated by natatory ciliae, (described as the outer divi- 

 sion of the limb, or future Jiagrum ;) and, 3d, a short rudiment of one of 

 the future hranchia. — Ann. of Phil. Feb. 1831. 



Respiration of the Sabella. — The filaments or processes placed round the 

 mouth in the Sabellaave generally regarded as respiratory organs. Dr Riippell, 

 however, maintains that they are merely tentacula, or organs of prehension. 

 According to him, the respiratory organs are seven sacs or cells on each side 

 of the animal, which open externally under the seven pairs of transverse 

 plates situate on the anterior and lateral part of the body. Two blood-vessels 

 go to each of these cells, viz. one from a dorsal, and another from an abdo- 

 minal vascular trunk, and, after ramifying minutely, anastomose together. 

 Dr R. has been able distinctly to perceive the alternate filling and emptying 

 of the vascular system of the cells, but he could not clearly make out from 

 which of the great trunks the blood is conveyed towards the respiratory cell, 

 nor to which of them it is returned. The direction in which the blood 

 moves, therefore, still remains to be investigated. — Isis, 1830, p. 611. 



Anatomical Structure of the Larva of (Estrus Equi. — In a memoir on the 

 internal structure of the larva of (Estrus Equi, by Professor Schroder van 

 der Kolk, we find the following : — 



The heart is fixed by ligaments at the posterior part ; anteriorly it 

 passes into a dilatation, and is fixed to the oesophagus, where it receives 

 nervous filaments. It sends off" many branches from its middle part, which 

 pass into the fat, ramifying on the walls of the adipose vesicles. A ramus 

 profundus, arising from the posterior part of the heart, is distributed among 

 the muscles, fat, and skin, and enters the heart again at its anterior part. 



The respiratory organs, situate at the posterior part of the body, consist 

 of small vesicles, more or less distended with air, and covered with two lips. 

 In their middle they have a small opening to the exterior, which can be 

 closed by muscular fibres. The vesicles are connected with a reservoir of 

 air, into which the tracheae open ; of the latter there are two large ones 

 belonging to the body generally, and four smaller, destined for the organs of 

 generation. When the tracheae were examined with a microscope magnifying 

 600 times, canals were discovered between the spiral filaments in their 

 parietes, which, when touched with a wetted hair pencil, seemed to absorb 

 water. The tracheae being examined in an animal which was opened alive, 



