Patten has examined the eyes of certain Decapoda, (Penaeus, Palaemon, Galathea, Pagurus), especially those of the first-named, in great detail. They have the typical structure given in note p. 452, ante. Pedicels are present in all; and in Galathea the two hypodermis cells corresponding to each corneal facet are modified into a contractile iris. In Branchipus Grubii Patten finds that the retinophorae are grouped in fours; the corneal cuticula is not facetted; the corneal hypodermis cells indefinite in arrangement. There are no pedicels, and the outermost set of bacillate retinulae absent. The somatic eyes of Euphausia are held by Sars to be luminous organs (Challenger Reports, xiii. 1885, Schizopoda, pp. 70-72). Patten considers that they have the essential structure of eyes. They possess an extremely thick laminated argentea behind the retina (cf. Mitth. Zool. Stat. Naples, vi. 1886, p. 685 et seqq.) and is a simple oval sac with two venous inlets and a short anterior aorta in Cladocera, some Ostracoda and Copepoda. It is long, thoracic in position in Amphipoda, abdominal in Isopoda; long and extending from the thorax into the abdomen in Stomatcpoda; short in Podophthalmata, where it lies in the thorax.
It lies in a pericardial sinus in Arthrostraca and Thorac-ostraca, with which it communicates by four paired ostia in Isopoda, three, but sometimes fewer in Amphipoda: by a large anterior pair and twelve smaller posterior pairs in Stomatopoda: and in the Podopthalmata by two pairs in My sis and three pairs in Decapoda. It usually gives off in the higher Crustacea one or more anterior vessels to the eyes and antennae, lateral to the glands of the mesenteron and posterior into the abdomen: and in the Stomatopoda the posterior part gives off thirteen lateral pairs of vessels. The system of vessels is especially well developed in Isopoda, where the heart gives off lateral vessels to the feet. The anterior aorta in Arthrostraca forms a peri-oesophageal vascular ring, from which in Isopoda a sub-neural vessel is continued backwards giving off lateral branches. The venous blood-spaces are limited by connective tissue in Arthrostraca and Decapoda. In the three parasitic Copepodan genera, Lernanthropus, Clavella, and Cyonus (= Congericola\ there is a special system of tubes containing a plasma with haemoglobin in solution. Respiration may be partly cutaneous as in Copepoda and Cirripedia. The bivalve shell of Cladocera, Ostracoda, and Neb alia is penetrated by the blood currents.
But in most Crustacea organs of respiration exist in the shape of appendages to the thoracic limbs (Branchiopoda, Cladocera, Leptostraca, Amphipoda, Decapoda); to the abdominal limbs (Stomatopoda); and sometimes to the wall of the thorax itself (Decapoda), though in this case it is probable that their position is secondarily acquired. The endopodite of the abdominal limbs, with the exception of the first pair which form an operculum, is respiratory, thin, and transversely folded in Isopoda: and in the genus Tylos belonging to this order a special process of the four anterior pairs of abdominal feet bears cross folds with linear slits, which lead to branched air sacs. The inner wall of the branchiostegite in Decapoda is very thin, and probably respiratory, and in some land crabs there are vascular growths of the branchial chamber. The exopodite of the thoracic limbs in many instances, e.g. Phyllopoda, or of the second maxilla (Decapoda), forms a broad plate which keeps a current of water in constant motion.
A rhythmical opening and shutting of the anus, admitting and expelling water, and thus constituting an anal respiration, takes place in Phyllopoda, Branchiopoda, and Cladocera, in Copepoda, Astacus, some Nauplii and Zoaeae, and is probably very general.
1 Unicellular salivary glands are often found in the epistoma, or when that structure is small as in Phronimidae, surrounding the oesophagus. See on Astacus, p. 185.
Excretory organs exist as the shell gland of Phyllopoda and Copepoda which opens close to the second maxilla: the antennary gland aborted in the two orders named during development, but persistent in Malacostraca, except Stomatopoda, and opening on the basal joint of the second antenna: and in Amphipoda as one or two glands generally said to open into the anterior end of the proctodaeum, but in reality it appears opening into the posterior end of the mesenteron (Spencer). A pair of glandular excretory (?) sacs open close to the anus in Stomatopoda (Claus).
The sexes are united in the majority of Cirripedia: and in the Cymo-thoidae (Isopodd) the sexual organ of the young animal is male, of the old, female in function. In a few Cirripedia minute cirriped-like 'comple-mental' males are found on the hermaphrodite animal. In Scalpellum vulgare the male is not like a Cirriped, nor is it in the few instances where the sexes are separate, e. g. Sc. ornatum. In many Copepoda the animal is sexually mature before it is adult. The sexual organs vary much in character; they may be paired or azygos, and if paired may be united across the middle line, e. g. Decapoda. They lie generally in the thorax, sometimes in the abdomen, e. g. Stomatopoda, Pagurus. The ducts are as a rule double, each with its separate aperture. This aperture is variously placed in the thorax or abdomen in Entomostraca 1, but in the Mala-costraca the male apertures open on the coxae of the last or eighth thoracic feet, the female on the coxae of the sixth, or else on the sterna of the corresponding somites. In the Cirripedia a long penis terminates the abdomen.
Accessory glands are rarely present, e. g. the gland which secretes the capsule of the ovisac in Copepoda. Receptacula seminis are occasionally found as in most Copepoda. Parthenogenesis occurs in Artemia and Apus among Branchiopoda; in the Cladocera (so-called summer ova) and in some Ostracoda (certain species of Cypris2). The offspring belong to the female sex. In Cladocera males are eventually produced, and this is probably the case in Ostracoda, in Artemia and Apus, but has not been proved to occur. The male generally differs from the female in size, e. g. in parasitic Copepoda where it is minute; in greater development of organs of special sense; in the presence of organs modified to retain the female; or of accessory efferent organs, e.g. the two first pairs of modified abdominal feet in Decapoda: and in the absence of special contrivances to retain the ova, e.g. brood lamellae, such as are attached to more or fewer of the thoracic limbs in Arthrostraca and Mysis.