This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
* Vide Ad. Brongniart, "Mem. sur le Limnadia," Mem. du Mus. 1820. 1 Straus-Durckheim, "Mem. sur le Daphnia," Mem. du Mus. 1820.
In the Pycnogonidae*, the aperture of the mouth is found at the extremity of the tubular proboscis which projects from the anterior part of the body of these remarkably-constructed animals, and which, from its general conformation, certainly reminds us more of the Acaridiform Arachnidans than of the Crustacean type of structure. The oesophagus is an extremely delicate and slender canal which passes directly backwards into the cephalothorax, where it at once expands into a central digestive cavity or stomach which occupies the centre of the body, and terminates posteriorly in a very narrow and rudimentary intestine.
(1112). From the circumference of the stomach are given off ten long caeca, the disposition of which is remarkable: of these, the two anterior are prolonged forwards to the pincer-like rudimentary foot-jaws or palpi, into the interior of which they penetrate for some distance; while the remaining four pairs, which are of great length, are continued in a similar manner into the locomotive or thoracic legs, extending almost to the end of the antepenultimate joint. The anal orifice of the intestine is situated, as usual, at the extremity of the very rudimentary abdomen.
(1113). When distended with fluid, these caeca may be observed to become constricted opposite to each articulation of the limb. Their structure is exceedingly simple; indeed, they seem to consist of a very thin diaphanous membrane, in which no trace of fibre is distinguishable, but which externally seems to be crusted over with a granular opake substance, sometimes presenting a violet or yellowish tint. These granulations are more thinly scattered over the stomach than over the caeca, and upon the intestine they are wanting altogether. The whole of this digestive apparatus, notwithstanding that its walls contain no perceptible fibres, is contractile, and floats freely in the general cavity of the body, being only retained in situ by a few delicate fraena; its different parts may be observed to have alternate movements of contraction and dilatation, driving, in undulations, first in one direction and then in another, the liquid which they contain. This liquid, which is quite transparent, hurries along with the materials in process of digestion.
These generally present themselves under the appearance of roundish or ovoid masses, about 1/40th of a millimetre in diameter, smooth and entirely without granulations during the earlier period of the digestive process; but as digestion advances, they may be seen to become decomposed into roundish granules that powerfully refract the light, and which are scarcely 1/300 th of a millimetre in size. The faeces seen in the intestine are entirely made up of these granules irregularly agglomerated together; and it is rare to find among them any traces of alimentary substances which are not entirely decomposed.
* Vide M. de Quatrefages, Ann. des Sci. Nat. 1844, iv. p. 72.
(1114). It has been stated above, that all that portion of the alimentary canal which intervenes between the oesophagus and the intestine is free, and floating loosely in the general thoracic cavity, which cavity is prolonged into the limbs extending beyond the terminations of the caeca; and in this cavity it is easy to distinguish the muscles subservient to locomotion, and which, more especially in the limbs, line, as it were, all the interior of the different joints, so that the digestive apparatus is evidently lodged in a great lacuna or cavity which occupies the entire thorax, and is prolonged into the claws. This lacuna is filled up with a transparent fluid, in which may be distinguished a great number of irregular transparent corpuscles, which appear to consist of agglomerations of smaller globules. The fluid is constantly agitated with irregular movements backwards and forwards, which are determined by the general movements of the animal, or by the alternate contractions and dilatations of the stomach and caeca, and which constitute all the circulation which is discernible in these creatures.
No organ is detectible specially appropriated to this function; heart and blood-vessels are alike wanting, the great lacuna above described taking the place of both, since the fluid which it contains is evidently the representative of the blood, or, rather, it is the blood itself. Neither are there any special organs appropriated to respiration, which is here evidently carried on by the general surface of the body, as we have found it to be in many of the Entomostracous Crustaceans.
(1115). The remarkable disposition of the alimentary canal so conspicuous in the Pycnogonidae, and which exists, to a greater or less extent, among the inferior tribes of various classes of animals, has been named by M.de Quatrefages "phlebenterism" from the circumstance that in the instance above given, and in many similarly-organized creatures, the intestinal ramifications supersede, to a greater or less extent, the functions of the circulatory, respiratory, and chyliferous systems of the higher animals.
(1116). The nervous system of the Pycnogonidae consists of a thoracic chain of ganglia, from which are derived the nerves supplying the limbs, and of a supra-oesophageal mass, giving off the optic nerves to form the minute ocelli that constitute the visual organs of these extraordinary creatures.