NO. 3 INSECT HEAD SNODGKASS II 



anterior open extremity is carried inward by an ectodermal invagina- 

 tion which becomes the stomodeum (C, Stom), the external opening 

 of which is the definitive mouth (Mth). The proctodeum and the 

 anus are formed later by a posterior invagination of the ectoderm, 

 and the proctodeum secondarily opens into the posterior end of the 

 stomach. At the anterior end of the preoral region of the body, or 

 prostomium, the ectoderm is thickened to form a sensory apical plate 

 (D, G, AP) beneath the sensory organs here located, and from it 

 ectodermal nerve tracts extend posteriorly in the body wall (F). 

 Typically, there is a pair of dorso-lateral longitudinal nerves (DNv), 

 and a pair of ventro-lateral nerves (VNv). The simple musculature 

 of the trochophore (E) is developed from mesenchyme tissue; the 

 epithelial mesoderm forms only the pair of mesoderm bands (D, Msd) 

 and a pair of nephridia (Nph) in the posterior part of the body. 

 This description of the trochophore is based mostly on that of Hats- 

 chek (1888-1891), from whose work the illustrations of figure 7 are 

 taken. 



The trochophore develops into the worm form of its parents by a 

 metamorphosis involving an elongation of its posterior end (fig. 9), 

 accompanied by a reduction of the cephalic swelling, until finally, in 

 the adult, the only dififerentiation in the head region is an anterior, 

 median prostomial lobe overhanging the mouth (fig. 10, Pst). The 

 prostomium usually bears the principal sensory areas or organs of 

 the worm, and a ganglionic nerve mass is differentiated from the 

 inner surface of its ectoderm, which becomes the supraoesophageal 

 ganglion, or brain, of the annelid. In the Polychaeta, the prostomium 

 may bear one or more pairs of eyes, and several pairs of sensory ten- 

 tacles (fig. 10). As the body of the young worm elongates, it be- 

 comes transversely segmented, the somites increasing in number 

 posteriorly as the segmented area lengthens. 



The young arthropod embryo, in its first definite form (fig. 8 A), 

 consists of a large head region, the so-called cephalic lobes (Pre), and 

 of a slender body (Bdy). The mouth (B, Stom) is situated on the 

 ventral surface of the cephalic enlargement. The proctodeal invag- 

 ination and the anus are formed, usually in a later stage, at the pos- 

 terior end of the body. 



The large-headed stage of the young arthropod embryo has a cer- 

 tain resemblance to the trochophore stage of the annelid larva ; but it 

 is probable that the similarity between the two forms has no genetic 

 significance, and that the size of the cephalic lobes in the arthropod 

 embryo is to be explained as an acceleration of development. Yet, 

 it is evident that the cephalic region of the arthropod embryo cor- 



