182 J. GRAHAM KERR. 



preserved specimens, to extend my investigations and to fill up 

 the obvious lacunae. 



I can hardly adequately express the obligation under which 

 I am to Mr Sedgwick for the generous gift by which he has 

 made these investigations possible and opened the way to what, 

 however poor its results are so far, has proved a study of 

 absorbing interest, and also for much kind advice and en- 

 couragement. To Mr Wilson also a word of thanks is due for 

 the care with which he has attended to the illustrations. 



II. The Body-cavity of Nautilus. 



It is now generally recognized that the body-cavity in the 

 higher Metazoa may be referred to either of two very distinct 

 types. The first of these, typically developed in Annelida and 

 Vertebrata, is lined by a definite characteristic epithelium, from 

 some of whose cells arise the genital products, while others 

 become the renal excretory cells. It appears at an early stage 

 in development as a more or less continuous space, and it 

 communicates with the exterior by apertures in the body-wall. 

 To a body-cavity of this type it is advisable to restrict the term 

 Ccdom. 



The second type of body-cavity is to be found in the 

 Mollusca and Arthropoda generally. It is part of the vascular 

 system, through it is pumped a continuous stream of blood by 

 the heart, and it does not communicate with the exterior. It 

 may be looked on as being formed by the expansion of the 

 terminal parts of the blood-vessels into large sinuses whose 

 walls have, to a greater or less extent, disappeared, giving rise 

 to a sponge-work more or less sparse according to the extent to 

 which this process has gone on. This type of body-cavity was 

 named by Sedgwick, Pseudocoel; by Lankester, Hcemocoel. 

 The word coelom has been used with such looseness that 

 Lankester 's term is perhaps to be preferred ; all the more so as 

 it specifies in itself one of the main characteristics of this form 

 of body-cavity. 



Occurring well-developed in Annelids, at least allied in all 



