ZOOLOGY. 



349 



by whicli it contrasts with all the three previously recognized families, 

 Von Kennel has lately proposed to isolate it as the type of an inde- 

 pendent one. This family {3IalacohdelUdw) is distinguished by "the 

 dermo muscular layer arranged in an external, circular or an internal 

 longitudinal layer ; the absence of spines to the proboscis ; the non- 

 development of cephalic grooves or lateral organs; the intestine simple 

 and forming several coils; the nerve-trunks free from the muscular 

 system and united posteriorly by an anal commissure, and the develop- 

 ment of a sucker at the posterior end." * 



CmETOGNATHA. 



KELATIONS OF THE CH^TOGNATHA. 



A form that derives considerable interest from the speculations that 

 have been entertained about it by the earlier naturalists, is the worm 

 group known as Sagitta, and for which the order Chaetognatha has 

 been instituted. An elaborate monograph of this order has been pub- 

 lished by Dr. Hertwig during the past year, and he has especially inquired 

 into the subject of its relationship with other types. + He recalls the 

 investigations of Kowalevsky and the application by Huxley of the 

 condition of the body cavity to taxonomy, and rehearses that in most 

 animals this ccelom is formed by a cleavage of the mesoblast (schizoccele), 

 and that in others— as in the Echinoderms, Brachiopods, and Amphiox- 

 us— the ccelom is developed from outgrowths of the endoblast (enter- 

 ocoele). It is the latter type that is manifested in the Chaetognatha. 

 The Chfetognatha are said to be most nearly related to the i^ema- 

 toids and Annelids, and the relations to the latter are claimed to be 

 very remarkable. In both " the enteron is invested in a fibrous enteric 

 layer, and is attached by mesenteries which completely divide the 

 ccelom into a right and left half. In both cases there are four bands of 

 longitudinal muscular fibers, the cells of which are derived from the 

 coelomatic surface. The two transverse septa of the Chajtognatha are 

 comparable to the numerous transverse septa in the Annelids, while, 

 lastly, in both groups the generative products are derived from cells of 

 the parietal layer of the mesoderm." The olfactory organ is on the 

 upper surface of the head near the eye and behind the supra-(Ksophageal 

 ganglion, and is unpaired and very simple in character. 



The nervous system is noteworthy, the chief ganglia and their nerves 

 being embedded in the epidermis, while some smaller cephalic ganglia 

 belong to the mesoderm; or, in other words, the nervous system is 

 differentiated into ectodermal and mesodermal parts. To the former 

 there are two central organs, the supra-oesophageal and the ventral 

 ganglia. In the latter are developed a lateral cephalic ganglion and 

 two other small ganglia. 



*Keimel (V.). Arieit. Zool. Zoot. Inst. Wiirzburg, vol. iv, p. 305, 1878. 

 t Hertwig (O.). Die Cbsetoguatlien. Studien zur Blatter iheorie, Heft ii, Jena, 1880, 

 6 pi. 



