TRANSACTIONS OF THE SECTIONS. 101 



truded from the orbit by a few turns of thread passed alternately behind and in front 

 of the eyeball in close contact with it. 



The result is, that the animal's eye is kept nearly immoveable, and that by direct- 

 ing the microscope to any point of the exposed surface, the circulation of the vessels 

 may be easily examined under a magnifying power of 200 diameters and upwards. 



Dr. Waller then proceeds to describe minutely the different parts of the eye, and 

 the distribution of the vessels over the cornea, the sclerotica,' the iris, and other deep- 

 seated parts of the eye. 



On the Mechanism of Respiration in the Family of Echinida. 

 By Thomas Williams, M.D. 



The author stated, that, after a very careful research upon the subject, he had 

 arrived at the conclusion that the mechanism of the breathing process in the Echi- 

 nidse dilFered in a radical manner from that which obtained in the Asteridie. In 

 the latter, the entire integumentary skeleton was perforated by minute orifices, 

 through which digital, membranous, ccecal processes protruded, and in and by 

 which the cavitary fluid was brought into contact with the exterior aerating 

 element. In the Echinidee, on the contrary-, the integumentary skeleton was 

 perforated only by the " ambulacral feet." ' The branchise in this family were 

 restricted to the membranous area which surrounded the mouth, around the 

 circumference of which they are disposed in a row. They differed in number and 

 structure in different orders, but in all they conformed to one type. They were 

 hollow-branched membranous processes, considerably larger in size than the 

 corresponding processes rising from the ligamentary surface of Jsterias ; and 

 communicated directly with the general cavity of the body. They bore an inti- 

 mate resemblance to the branchiae of the SipimculidcB. TheEchinidae differed from 

 the Sipunculidce in being totally destitute of every provision along the general sur- 

 face of the body which could aid in the office of respiration. The relation which 

 was thus established between the branchiae and the cavitary fluid in the Echinidw, 

 was a strong presumptive ground for the belief that a blood-vascular system did 

 not exist at all in this family of Echinoderms. 



On the Fluid System of the Nematoid Entozoa. By Thomas Williams, M.D. 



In this communication the author mentioned the leading facts: — 1. That espe- 

 cially in the genus Ascaris the peritoneal cavity was occupied by a peculiar vesicular 

 tissue, opening on the integumentary exterior, which appeared to be adapted to 

 absorb fluid from without. 2. That in some species it almost entirely filled up that 

 space which in the Annelids was/ree, and occupied by an oscillatory fluid; and, 3. 

 That in the Nematoid Entozoa there did not exist any trace whatever either of a 

 blood-vascular or a water-vascular system. 



Miscellaneous. 

 On the Variation of Species. By the Rev. L. Jenyns, M.A., F.L.S. 



The object of this paper was to draw the attention of the Natural History Section 

 of the British Association to the importance of collecting all the facts already known, 

 or which might be obtained by further researches, connected with the variation of 

 species. The subject was stated to be one, which, more than almost any other, 

 deserved the consideration of naturalists at the present day. Reference was made to 

 the many complaints which may be found in the works of different authors, as to the 

 difficulty of determining what is and what is not a species, as also to the excessive 

 multiplication of species by some naturalists, and their too ready disposition to over- 

 rate the value of those slight differences, by which many of these so-called species are 

 distinguished. It was thought that a very large number would probably prove to 

 be merely local races, originally derived from one stock, their differential characters 



