498 



NA TURE 



{March 22, 1888 



Dr. Derham also supplies what has long been wanted, a scien- 

 tific hydrometer having a succession of poises to continue the 

 series the indications of which are also specific gravities. It is 

 well known that, in order to effect this, the increment to the 

 total bulk of the instrument with each successive poise should 

 be the bulk of the graduated stem. Bates's saccharomater is a 

 more or less successful mechanical adaptation of this require- 

 ment. But it had escaped previous inventors that, in order to 

 perfectly satisfy the conditions of the problem, the specific 

 gravities of the successive poises should bear an exactly defined 

 relation to the specific gravities to be indicated by the in- 

 strument. The principle upon which the calculation of the 

 hydrometer is based is that — 



weight -r 



^— = specific gravity. 



bulk 



Let W = weight of hydrometer ; B = bulk of hydrometer ; 

 G = initial specific gravity of the instrument ; g = specific 

 gravity of any poise ; a = the number of degrees of gravity 

 indicated in the length of the stem ; and unity = bulk of 

 graduated stem ; then, since the bulks of the poises must be 

 multiples of the bulk of the graduated stem, according to their 

 position in the series, 



n = bulk of «th poise. 

 ng = weight of „ 



By the definition of specific gravity, 



= G H-a, 



whence 



W = BG, and ^^ - 

 B - I 



and Ba = G -f a. 



Ag-'^ifij generally, with «th poise attached, 

 BG + ng ^ ^ 

 B -f « 



whence ^ = 2G -f (« -f 1)^-. 



And if the hydrometer were intended to indicate gravities 

 frorn 780 to i-ooo, the value of the stem being -020, and the 

 initial specific gravity accordingly of each range '800, '820, '840, 

 &c., the successive specific gravities of the poises would be i-6o' 

 I '62, 1-64, &c. ' 



THE CCELOM AND THE VASCULAR SYSTEM 

 OF MOLLUSC A AND ARTHROPODAy ' 



'T'HE object of the author was to establish the fact that the 

 system of blood-containing spaces pervading the body in 

 Mollusca and in Arthropoda was not, as sometimes (and indeed 

 •usually) supposed, equivalent to the coelom or perivisceral space 

 of such animals as the Chretopoda and the Vertebrata, but was 

 in reality a distended and irregularly swollen vascular system — 

 the equivalent of the blood-vascular system of Chretopoda and 

 Vertebrata. Flence he proposed to call the body-spaces of 

 Mollusca and Arthropoda "haemocoel," in contradistinction to 

 "coelom." It had been held by previous investigators that in 

 Mollusca and Arthropoda the coelom and the vascular system 

 were united into one set of spaces — whether by a process of 

 gradual fusion, or owing to the fact that the two systems had 

 never been differentiated from a common original space repre- 

 senting them both in the ancestors of these two great phyla, 

 the author stated that he had been led to the view which 

 he now formulated by his discovery of distinct spaces in both 

 Mollusca and Arthropoda, which appear to be the true ccelom, 

 and are separate from the swollen vascular system. 



In Mollusca the pericardial space is the chief representative of 

 coelom. It is usually taught that the pericardium of Mollusks 

 contains blood, and is in free communication with veins; but 

 the author had succeeded in showing by observations on 

 the red-blooded Solen legumen (already published, Zoolog. 



' Abstract of a Paper read in Section D, at the Manchester meeting of the 

 Bntish Association, by Prof. Ray Lankester, F.R.S. 



Anzciger, No. 170, 1884), and by more recent careful investiga" 

 tion of Anodonfa cygnca, Pa:clla vtilgata, and Helix aspena, 

 that the pericardium has no communication with the vascular 

 system, and does not contain blood. The perigonadial spaces 

 (so-called generative glands) and the pericardial space (which 

 has arborescent tubular outgrowths in some Lamellibranchs 

 forming Keber's organ) are, then, the coelom of the Mollusca. It 

 is quite distinct from the ha:mocnel. In Cephalopods, and in the 

 archaic Gastropod Neomenia, the pericardial and perigonadial 

 crjelomic i-emnants are continuous, and form one cavity. There 

 is strong reason to believe that in ancestral Mollusks the haemocoel 

 was more completely tubular and truly vasiform than it is in 

 living Mollusks. In the later Mollusks the walls of the vessels 

 have swollen out in many regions (especially the veins), and have 

 obliterated the coelom, which has shrunk to the small dimensions 

 of pericardium and perigonadium. There are, however, many 

 Mollusks with complete capillaries, arteries, and veins, in certain 

 regions of the body. These had been recently studied by the 

 author by means of injections, and by silver impregnation, and 

 drawings illustrative of them were exhibited to the Section. 



With regard to the Arthropoda, Prof. Lankester formulated 

 the same view, viz. that the ancestral blood-vessels have swollen 

 and enlarged, especially the veins, so as to form large irregular 

 spaces, which have blocked up and so obliterated the previously 

 existing coelom. Nevei'theless the coelom still persists in some 

 parts of the Arthropod body quite separate from the swollen 

 blood-vascular system. It persists as the tubular generative 

 glands (peri'gonadii;m), and also as a system of small spaces 

 (lymph-system) in the connective-tissue of Astacus and of Limulus, 

 and as the internal terminal vesicle of the green glands and other 

 nephridia present in various Arthropoda. Prof. Lankester stated 

 that he had been led to this view with regard to the vascular 

 system and ccelom of the Arthropoda by the results of his histo- 

 logical investigations on the vascular system and connective-tissues 

 of Astacus and Limulus, and by the results obtained in his 

 laboratory by Mr. GuUand in studying the development of the 

 nephridial "coxal gland" of Limulus (already published, with 

 note by Prof. Lankester, in the Quart. Joiini. Micr. Sci., 1885, 

 vol. x>:v. p. 515). He had also been led to this view by the 

 attempt to explain theoretically the origin of the peculiar structure 

 of the Arthropod's heart and blood-holding pericardium. 



The Arthropod's heart and pericardium are absolutely peculiar 

 to the group, and characteristic of all its members — even of 

 Peripatus. The author had asked himself how the existence 

 of a tubular heart with paired valvular apertures in each segment 

 of the body — lying within a blood -holding sac — could be ex- 

 plained. He conceived that it might best be explained by that 

 tendency of the veins to dilate and to form irregular large blood- 

 sinuses, which on other grounds we have reason to consider as a 

 structural tendency of Arthropods. Each pair of valvular aper- 

 tures in the Arthropod's heart represents a pair of distinct 

 tubular veins which in the ancestors of the Arthropoda brought 

 blood to the heart from the gill<. These veins have dilated, and 

 their adjacent walls have been absorbed, so that we now have, 

 instead of a series of veins, a great continuous blood-sinus on 

 each side of the heart or dorsal vessel. 



Capillaries of the finest dimensions were shown by Prof. 

 Lankester to exist in certain parts of Astacus and of Limulus. 

 In studying these he had come across the remnants of coelom. 

 Between the capillaries and unconnected with them — in the 

 connective-tissue of both Astacus and Limulus — is a system 

 of spaces containing a coagulable fluid. (These spaces were de- 

 scribed and figured in Limulus in 1884 by Prof. Lankester in the 

 Quart. Journ. Micr. Sci.) It is into this system of spaces that 

 the tubular nephridium which becomes the coxal gland of 

 Limulus opens. Hence these spaces are remnants of the ccelom, 

 elsewhere blocked up and obliterated by the swollen veins which 

 form the hseniocoel. The tubular generative glands of Arthro- 

 pods are to be explained as perigonadial coelom communicating 

 with the exterior through modified nephridia. Beddard's dis- 

 covery of such a condition of the ovary and oviduct in the earth- 

 worm Eudrilus is confirmatory of this explanation. 



The views which had been thus arrived at by Prof. Lankester 

 and very briefly indicated in the note in the Quart. Journ. Micr, 

 Sci., 1885, p. 515, have received a startling and demonstrative 

 confirmation in Sedgwick's brilliant results as to the development 

 of coelom and haemocoel in Peripatus, published in the Quart. 

 Journ. Micr. Set., February 1888, and announced early in 1887 

 to the Cambridge Philosophical Society. 



