CIRCULATION IN ANNULOSA. 697 



the head, stomach, liver, and caudal extremity ; from the lacunae or spaces 

 between the tissues and organs, the circulating fluid is collected by veins, which 

 end in sinuses placed above the ventral surface ; from these, it is conveyed, by 

 distinct vessels, into the gills, there being sometimes contractile dilatations at 

 the base of those organs ; having passed through the gills or branchiae, it is 

 then returned, by the so-called branchio-cardiac veins, to the large venous sinus 

 inclosing the contractile heart, into which it enters through valved apertures, 

 and is then propelled into the systemic arteries already described. ~No cilia 

 ever exist in the interior of any part of the circulating system of the highest 

 or Arthropodous Annulosa. In most other Crustacea, the plan of the circu- 

 lating apparatus, is the same as in the lobster ; but in certain of the lowest 

 forms, the simple heart is replaced by an elongated, and sometimes by a seg- 

 mented, contractile vessel, or dorsal vessel, provided with lateral valved open- 

 ings, by which the blood enters from an inclosing venous space or sinus. In 

 Insects, the circulatory system presents, as has already been seen in so many 

 instances, modifications adapting it to special forms of the respiratory appa- 

 ratus. In them, there no longer exist either lungs or gills, but tracheal re- 

 spiratory tubes ramified through every part of the body. The heart is replaced 

 by a numerously segmented dorsal vessel, provided with valves between each 

 segment, contracting rhythmically from behind forwards, and having numer- 

 ous lateral valved openings, through which the circulating fluid, returned from 

 all parts of the body, enters from an inclosing so-called pericardial, but really 

 venous, space or sinus. The corpusculated fluid or blood, propelled from the 

 anterior extremity of the dorsal vessel, passes, however, not so much into dis- 

 tinct vessels, as into channels, lacunas, or spaces, in and between the various 

 organs and tissues of the body, and so comes into relation with the universally 

 diffused tracheal tubes, and then again finds its way back to the pericardial 

 venous sinuses. The chief modification of this system, seen in the Myriapoda 

 and Spiders, consists in the comparative length or shortness, the equal devel- 

 opment, and the more or less frequent segmentation of the valved dorsal ves- 

 sel, in accordance with the consolidation or multiplication of the segments. 

 Thus, in the Geophilida, the number of contractile segments is upwards of 

 150 ; in lulida, as many as 75 ; in Scolopendrida, 15 to 21 ; all are separated 

 by well-formed valves. A small ventral trunk also exists in some Myriapoda. 

 In the Arachnida, the dorsal vessel sometimes consists of only four or five 

 closely packed chambers. These segments, with their interposed valves, are 

 found only in the body of the perfect Insect, and usually number eight or less ; 

 in the thorax, the dorsal vessel has no valves, and, in front, it ends in a fine 

 vessel, from which a few branches have been traced. In the microscopic 

 Arachnida, such as the acari, there are no proper vessels, but the nutritive 

 fluid contained in the perivisceral chamber, is moved by means of contractions 

 of the muscles of the intestines and the skin. The scorpion presents the un- 

 usual condition of possessing a distinct abdominal vein, proceeding from the 

 tail, and giving branches to the pulmonary sacs. 



In the Annelida, the best anatomists agree in doubting the existence of 

 either a contractile chamber or heart, or of a contractile dorsal vessel ; the 

 only cavities possessed by them, analogous to the more perfectly developed 

 circulatory apparatus of the Arthropodous Insects, Myriapods, Spiders, and 

 Crustacea, are the perivisceral spaces, lacunoe, and channels, in which a slightly 

 corpusculated fluid has been found. It is in this Class, including the various 

 marine and terrestrial Worms and the Leeches, that those remarkable ramified 

 vessels are met with, extending into every segment and portion of the body, 

 named the pseudo-hcemal vessels, which contain, sometimes a colorless, though 

 usually a red-colored fluid, often corpusculated, but the color of which is not 

 dependent on the corpuscles. These vessels are at one or many parts, dilated 

 and contractile, and, at certain parts, lined with cilia. They always commu- 

 nicate, at some point, by a tubular stem with the exterior ; a principal trunk 

 on the dorsal aspect of the body, has been regarded as the representative of 

 the true dorsal vessel of the higher Annulosa ; but their homologies are believed 

 to be rather with the so-called water-vessels of the still more lowly organized 

 Annuloida, than with the non-ciliated vascular apparatus of the Arthropodous 

 Annulosa. 



