Fig. [30].—Haemocera danae, × 40. A, Side view ♀; B, ventral view ♂. Ant.1, 1st antenna; e, eye; ov, ovary; ovd, oviduct; St, stomach; Th.1, 1st thoracic appendage; Th.5, 5th thoracic segment; vd, vas deferens. (After Malaquin.)
Fig. [31].—Free-swimming Nauplius larva of Haemocera danae; Ant.1, Ant.2, 1st and 2nd antennae; e, remains of eye; Md, mandible. (After Malaquin.)
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).
Fig. [32].—Later stages in the development of Haemocera danae. Abd, Abdomen; Ant.1, Ant.2, 1st and 2nd antennae; ch, chitinous investment; e, eye; Ect, ectoderm; En, endoderm; Mes, mesoderm; Mes & en, mesoderm and endoderm; R, rostrum; St, mouth and stomach; Th, thoracic appendages. (After Malaquin.)
Fig. [33].—Side view of Doropygus pulex, ♀, × 106. Abd.1, 1st abdominal segment; Ant.1, 1st antenna; b.p, brood-pouch; Th.1, 1st thoracic appendage; Th.4, 4th thoracic segment. (After Canu.)
Fam. 5. Ascidicolidae.[[46]]—Although the members of this family, which live semiparasitically in the branchial sac or the gut of Ascidians, betray their Ampharthrandrian nature by the sexual differences of their first antennae, only two genera, Notodelphys and Agnathaner, possess true prehensile antennae. According as the parasitism is more or less complete, the buccal appendages either retain their masticatory structure or else become reduced to mere organs of fixation. In Notodelphys both sexes can swim actively and retain normal mouth-parts; they live parasitically, or perhaps commensally, in the branchial cavities of Simple or Compound Ascidians, feeding on the particles swept into the respiratory chamber of the host. They leave their host at will in search of a new home, and are frequently taken in the plankton.
Doropygus (Fig. [33]), a genus widely distributed in the North Sea and Mediterranean, also inhabiting the branchial sac of Ascidians, is more completely parasitic, and the female cannot swim actively. Forms still more degraded by a parasitic habit are Ascidicola rosea (especially abundant in the stomach of Ascidiella scabra at Concarneau), in which the female has lost its segmentation, the mouth-parts and thoracic legs being purely prehensile, and various species of Enterocola, parasitic in the stomach of Compound Ascidians, in which the female is a mere sac incapable of free motion, while the male preserves its swimming powers and a general Cyclops-form (Fig. [34]). We have here the first instance of the remarkable parallelism between the degree of parasitism and the degree of sexual dimorphism, a parallelism which holds with great regularity among the Copepoda, and can be also extended to other classes of parasitic animals.