48 SUMMARY OF CURRENT RESEARCHES RELATING TO 
as an unpaired diverticulum. In correlation with the excessive deve- 
lopment of the cephalic lobes we have the preoral part of the enteron, 
which Bateson took for the notochord, especially well developed. As 
in Balanoglossus , so in Ampliioxus, we find a pair of enteric outgrowths 
in the anterior part of the archenteron ; the left one becomes, later on, 
connected with the external medium in both forms. Of the three out- 
growths from the archenteron in Crinoids the unpaired one corresponds 
to the cephalic coelom of the Enteropneusta, and is converted into the 
ambulacral ring. 
According to Hatschek we have, in the embryo of Amphioxus , to do 
with two divisions of the coelom (myocoel and splanchnocoel) ; in the 
Enteropneusta we find this division only in the collar-segment, where 
the myocoel (Bateson’s perihsemal cavity) is very closely connected 
with the longitudinal musculature. The pulsating vesicle of Tornaria 
(Bateson’s proboscis-gland) is nothing else than a division of the coelom, 
and probably represents the myocoel of the proboscis-segment. 
The vascular system of B. Meresclikowskii is formed by two trunks — 
a dorsal and a ventral — which lie between mesenteries, and probably pass 
into one another at either end of the body. This arrangement obtains 
in Ampliioxus and Synapta, as well as in Annelids and Nemertines, 
The anterior part of the enteron of Balanoglossus is divisible into a 
suprabranchial and a suboesophageal ; the latter is reduced in B. Meresch- 
Icowshii to a groove, which is represented by the endostyle of Tunicates, 
the hypobranchial groove of Ampliioxus , and the thyroid evagination 
of Ammocoetes. Notwithstanding various attempts to homologize other 
structures with them, the gill-clefts remain characteristic of the Chordata. 
The gonads of Annelids, Nemertines, Enteropneusta, Ampliioxus , and 
the hypothetical ancestors of Echinoderms, are formed on the same type, 
and it can scarcely be doubted that the genital orifices of Nemertines 
and Enteropneusta should be homologized with the ectodermal part of 
the segmental organs. 
The nervous system of Tornaria seems to consist of the cejfhalic 
ganglion beneath the eyes, whereof no trace is left in the adult Balano- 
glossus ; the nervous tube of this creature corresponds to that of the 
Chordata ; the anterior neuropore corresponds to the ciliated pit of the 
Amphioxus- larva and the ciliated duct of the neural gland of Tuni- 
cates. In no way can the dorsal and ventral nerve-trunks of Balano- 
glossus be compared with the nerve-trunks of Holothurians. 
All the Bilateria may be arranged in four groups, according to the 
structure of the nervous system : — 
(1) Gastroneura — with a cephalic ganglion and two ventral trunks. 
(2) Tetraneura (Mollusca) — with a cephalic ganglion, two ventral, 
and two lateral trunks. 
(3) Cycloneura (Echinodermata) — with no cephalic ganglion, but 
with an oesophageal ring and five radial trunks. 
(4) Notoneura (Enteropneusta and Chordata) — with no cephalic 
ganglion, but with a dorsally placed nerve-tube. 
Fecundation of Hydatina senta.* — M. Maupas has been able to 
convince himself absolutely that the “ winter eggs ” of this Rotifer are 
Comptes Rendus, xci. (1890) pp. 310-2. 
