ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
331 
though rudimentary, and is lodged in a special pericardium. It has the 
form of a rounded, thin-walled bag, and is not divided into two 
chambers. This simple structure, the complete absence of vessels with 
proper walls and of renopericardial orifices, are signs of degeneration. 
The heart is nothing more than a saccular invagination into the lumen 
of the pericardium of part of the dorsal pericardial wall. The blood- 
corpuscles enter the heart by narrow clefts, which lie between the 
stomach and the dorsal pericardial wall, and they leave it by similar 
spaces between the wall and the kidney. The minute structure of the 
wall of the heart is the same as that of the pericardium, and in both 
there are numerous muscular filaments set circularly and parallel to 
one another. 
5. Lamellibranchiata. 
Blood of Lamellibranchs.* —Dr. H. Griesbach has investigated the 
blood of fifty-five bivalves with the following result. The red pigment 
of several (e. g. Poromya granulata , Solen legumen , Tellina planata, Area 
Nose , Pectunculus glycimeris ) is haemoglobin, or very nearly allied to it. 
The pigment is diffused in special disc-like or spherical cells which 
have a distinct membrane, a finely striated structure, a nucleus and 
nucleolus. The leucocytes are clear or granular, and consist of a spongy 
framework with more unstable material in the meshes. When alive they 
do not take up pigment injected into the blood, until they have lost their 
normal characteristics. In the intact cells there are no vacuoles. The 
unstable material forms long pseudopodia, which are insheathed for some 
distance by the firmer spongy substance. By these processes the leuco- 
cytes are never united to one another. The contractile substance is 
limited by a “ plasma-membrane,” which is readily affected by abnormal 
conditions. All the leucocytes have a distinct nucleus, which lies in a 
“ free space,” contains two chemically different substances, and has no 
definite reticulate structure, nor any nuclear membrane. No processes 
of division were observed. The blood of some Lamellibranchs contains 
crystals, which effervesce when an acid is added. The varied movements 
of the leucocytes as observed in artificial conditions are in great part 
the results of abnormal physical and chemical influences. When water 
enters the system the pigmented and unpigmented cells are abnormally 
affected — a fact which Griesbach uses as an argument against the sup- 
position that water may enter the blood during normal life. 
Lepton squamosum.t — The Rev. Dr. A. M. Norman has an inter- 
esting note on this Mollusc, which he shows to be a commensal. While 
digging at Salcombe he observed that wherever the long passages formed 
by Gebia stellata were still occupied by the living Crustacean, the Lepton 
was to be found near. The burrows of the Gebia are lined with an 
ochreous-coloured slimy deposit, upon which it is probable that Lepton 
feeds. The geographical range of the Crustacean and the Mollusc 
appears to be the same, and similar observations by Stimpson on the 
Floridan Lepton loripes confirm the view that we have here to do with 
a case of commensalism ; as does also a specimen found at Puget Sound.J 
* Arch. f. Mikr. Anat., xxxvii. (1891) pp. 22-99 (2 pis.). 
t Ann. and Mag. Nat. Hist., vii. (1891) pp. 276-8. % T. c., p. 387. 
