A Vegetative Crustacean ¹

Abstract

EVERY text-book of zoology mentions, as one of the stock examples of degeneration, the curious cirripede, Sacculina, which lives as a parasite on crabs. Through the researches of Prof. Delage, more recently confirmed and extended by Mr. Geoffrey Smith, its life-history is now well known. After passing through free-swimming larval stages closely comparable with those of the normal barnacles, Sacculina attaches itself to its host and becomes endoparasitic, developing a system of branching roots which ultimately permeate all the organs of the crab. These roots radiate from a central mass of cells within which the sac-like body is differentiated, to emerge later on the surface beneath the crab's abdomen. While Sacculina is, as a rule, solitary, some related forms occur in considerable numbers on a single host. The development of these has hitherto been something of a puzzle, since all the individuals found on one host are approximately in the same stage of development, and simultaneous infection by a swarm of larvae seems improbable on account of the rare occurrence of the parasites. Mr. Geoffrey Smith, in 1906, made the suggestion that these gregarious parasites might be produced by some process of polyembryony or budding in the endoparasitic stage. He based the suggestion on the fact, observed by Delage and confirmed by himself, that the developing Sacculina may, as a rare exception, produce twin rudiments of the body within a single embryonic mass.