=Fam. 4. Monstrillidae.=[45]—These are closely related to the Harpacticidae. The members of this curious family are parasitic during larval life and actively free-swimming when adult. There are three genera, _Monstrilla_, _Haemocera_, and _Thaumaleus_. The best known type is _Haemocera danae_ (often described as _Monstrilla danae_). In the adult state (Fig. 30) there are no mouth-parts; the mouth is exceedingly small and leads into a very small stomach, which ends blindly, while the whole body contains reserve food-material in the form of brown oil-drops. The sole appendages on the head are the first antennae; but on the thorax biramous feet are present by means of which the animal can swim with great rapidity. This anomalous organisation receives an explanation from the remarkable development through which the larva passes. The larva is liberated from the parent as a Nauplius with the structure shown in Fig. 31; it does not possess an alimentary canal. It makes its way to a specimen of the Serpulid worm, _Salmacina dysteri_, into the epidermis of which it penetrates by movements of the antennae, hanging on all the time by means of the hooks on the mandibles. From the epidermis it passes through the muscles into the coelom of the worm, and thence into the blood-vessels, usually coming to rest in the ventral blood-vessel. As the Nauplius migrates, apparently by amoeboid movements of the whole body, it loses all its appendages, the eye degenerates, and the body is reduced to a minute ovoid mass of cells, representing ectoderm and endo-mesoderm, surrounded by a chitinous membrane (Fig. 32, A). Arrived in the ventral blood-vessel it begins to grow, and the first organ formed is a pair of fleshy outgrowths representing the second antennae (Fig. 32, B), which act as a nutrient organ intermediary between host and parasite. The adult organs now begin to be differentiated, as shown in Fig. 32, C, from the undifferentiated cellular elements of the Nauplius, the future adult organism being enclosed in a spiny coat from which it escapes. At this stage it occupies a large part of its host’s body, lying in the distended ventral blood-vessel, and it escapes to the outside world by rupturing the body-wall of the worm, leaving behind it the second antennae, which have performed their function as a kind of placenta. Malaquin, to whom we owe this account, makes the remarkable statement that if two or three Monstrillid Nauplii develop together in the same host they are always males, if only one it may be either male or female. The only parallel to this extraordinary life-history is found in the Rhizocephala (see pp. 96–99). [Illustration: