In the early summer of 1915 it was' noticed that many medusæ brought in with the tow-nettings contained larval Pycnogonids in the manubrium and at the junction of manubrium and stomach. The medusæ specially noticed to contain them were Obelia sp., Cosmetira pilosella, Turris pileata, Stomotoca dinema and Phialidium hemisphericum. By far the greater number were in Obelia, although many were in Phialidium hemisphericum and Cosmetira pilosella. They were extremely abundant in June, after that became scarcer, and finally disappeared by October. On examination they were seen to be larval stages of Anaphia petiolata (Kröyer), a Pycnogonid common in Plymouth Sound. The older larvæ sometimes were seen to cast their skins, so that the species could be easily recognised, although the fourth pair of walking legs were not fully developed. This is evidently the species described by Dogiel (1913) as Anoplodactylus pygmœus, the life history of which he traces from its first entry into the Obelia hydroid to the older stages when it is ready to leave its host. The form he refers to as Anoplodactylus petiolatus occurring in cysts in Coryne with Phoxichilidium femoratum must be some other species, as his figures prove clearly that it differs from A. pygmœus, and also the colour is totally different (a bright pink, while the present form is a pale yellow). Dogiel believes he has proved that Anoplodactylus petiolatus and A. pygmœus are different species from the difference in their life histories, and it is evident that he is dealing with two different species, but his A. petiolatus cannot be the same as our form, which is certainly identical with his A. pygmœus, and shows that Sars (1891) and Norman (1894) were right in regarding A. pygmœus as the young form of A. petiolatus (Kröyer).