1. Individuals of Cryptochiton stelleri, as representatives of the class Placophora, distributed inulin in blood volumes yielding a mean of 43.8% of the wet body weight without shell. Mean cellular water was estimated to be 76%.

2. The two opisthobranch gastropods examined were found to have very large mean blood volumes. Aplysia californicus distributed haemoglobin in 76.2% and mild silver proteinate in 73.1% of the wet body weight. Inulin was distributed in a significantly larger space averaging 79.3% of the body weight, which probably included pericardial and renal spaces. Archidoris sp. distributed inulin in 65.4% of the wet body weight. Mean cellular waters were 74-79% in Aplysia and 83% in Archidoris.

3. Two pulmonate gastropods were studied with inulin which was distributed in a mean space 40.3% of the wet body weight without shell of Achatina fulica, and 36.6% of the wet body weight in Arion ater. The computed cellular waters were 77 and 79%, respectively.

4. Excellent agreement was shown between a marine pelecypod, Mytilus californianus, and a fresh-water pelecypod, Margaritana margaritifera, of similar body size and form. Inulin in the former was distributed in 50.8%, and in the latter inulin and T-1824 in 49% of the wet body weight without shell. The cellular water contents were 80 and 76%, respectively.

5. In a single representative of the Cephalopoda--Octopus hongkongensis, it was possible to demonstrate with T-1824 and with HgS a blood volume averaging 5.8% of the wet body weight, constituting a fluid space distinctly different from the tissue fluid space. Inulin was distributed in the entire extracellular space amounting to 28% of the wet body weight. The cell water which was calculated from the mean values obtained was 77%.

Supported in part by a contract with the Office of Naval Research and in part by funds for the support of research in biology and medicine under Initiative 171 of the State of Washington. Work done at the Kerckhof Marine Biological Laboratory of the California Institute of Technology, Marine Biological Laboratory of the University of Hawaii and the Friday Harbor Laboratories of the University of Washington.
 Contribution No. 98 of the Marine Biological Laboratory.