Development of Organs.
The alimentary tract after the obliteration of the blastopore forms a closed sack, which becomes subsequently placed in communication with the exterior by the stomodæal invagination. The liver is formed as a pair of dorsal outgrowths of the mesenteron. From Brooks’ observations on Lingula it would appear that the primitive mesenteron forms the stomach of the adult only, and that the intestine grows out from this as a solid process: this eventually meets the skin, and here the anus is formed. In the Articulata the mesenteron is aproctous.
The origin of the body cavity as paired archenteric diverticula has already been described. Its somatic wall becomes in Lingula ciliated, and its cavity filled with a corpusculated fluid, as in many Chætopods. It is eventually prolonged into the dorsal and ventral mantle lobes as a pair of horn-like prolongations into each lobe, which communicate with the body cavity by large ciliated openings. Some incomplete observations of Brooks on the development of the nervous system in Lingula shew that it arises in the embryo as a ring round the œsophagus with a ventral sub-œsophageal ([fig. 138] q), and two lateral ganglia, and two dorsal otocysts. The ventral ganglion is formed as a thickening of the epiblast, with which it remains in continuity for life. The remainder of the ring grows out from the ventral ganglion as two cords, which gradually meet on the dorsal side of the œsophagus.
General observations on the Affinity of the Brachiopoda.
The larva of Argiope, as has been noticed by many observers, has undoubtedly very close affinities with the Chætopoda. It resembles, in fact, a mesotrochal larval Chætopod with provisional setæ (vide Chapter on Chætopoda). Lacaze Duthiers’ observations point to the lobes of the larva not being true segments, and certainly the mesoblast does not in the embryo become segmented as it ought to do were these lobes true segments. If this view is correct the larva is to be compared to an unsegmented Chætopod larva. In Rhynchonella, however, indications of two segments are afforded in the adult in the two pairs of segmental organs.
Though the larval Brachiopod resembles a mesotrochal Chætopod larva, it does not appear to resemble the trochosphere larvæ so far described, or the more typical larvæ of the Chætopoda, in that the ring of tentacles, which is probably, as already mentioned, derived from the ciliated ring shewn in [fig. 137], is post-oral, and not præ-oral. The ring of tentacles is like the ring in Actinotrocha (the larva of Phoronis) amongst the Gephyrea. Although there is no doubt a striking resemblance between the tentacular disc of a larval Brachiopod and the lophophore of a Polyzoon, which has been pointed out by Lankester, Morse, Brooks, etc., their homology is rendered, to my mind, very doubtful (1) by the fact that the lophophore is præ-oral in Polyzoa[133] and post-oral in Brachiopoda; and (2) by the fact that the concave side of the lophophore is turned in nearly opposite directions in the two forms. In Brachiopods it is turned dorsalwards, and in phylactolæmatous Polyzoa ventralwards.
The view of Morse, that the Brachiopoda are degraded tubicolous Chætopods, is not so far supported by any definite embryological facts. The development of the tentacular ring as well as its innervation from the sub-œsophageal ganglion prohibit us, as has been pointed out by Gegenbaur, from comparing it with the tentacles of tubicolous Chætopoda.
Bibliography.
(325) W. K. Brooks. “Development of Lingula.” Chesapeake Zoological Laboratory, Scientific Results of the Session of 1878. Baltimore, J. Murphy and Co.
(326) A. Kowalevsky. “Development of the Brachiopoda.” Protocol of the First Session of the United Sections of Anatomy, Physiology, and Comparative Anatomy at the Meeting of Russian Naturalists in Kasan, 1873. (Russian.)
(327) H. Lacaze Duthiers. “Histoire de la Thécidie.” Ann. Scien. Nat. etc. Ser. 4, Vol. XV. 1861.
(328) Morse. “On the Early Stages of Terebratulina septentrionalis.” Mem. Boston Soc. Nat. History, Vol. II. 1869, also Ann. & Mag. of Nat. Hist., Series 4, Vol. VIII. 1871.
(329) —— “On the Embryology of Terebratulina.” Mem. Boston Soc. Nat. History, Vol. III., 1873.
(330) —— “On the Systematic Position of the Brachiopoda.” Proceedings of the Boston Soc. of Nat. Hist., 1873.
(331) Fritz Müller. “Beschreibung einer Brachiopoden Larve.” Müller’s Archiv, 1860.
[128] The classification of the Brachiopoda adopted in the present chapter is shewn in the subjoined table: