The monocytes of lampreys [Lethenteron camtschaticum (adult), L. reissneri (adult), L. hattai (adult, ammocoetes)] were round or oval, low nucleus/cytoplasm ratio, and have striated (two-layered) granules (MoG). The MoG consisted of a basophilic inner layer (L0) and a chromophobic outer layer (L1). The L0 were positive for acid phosphatase, α-naphthyl acetate esterase, naphthol AS-D chloroacetate esterase and Sudan black B. Also, the L0 showed orthochromatic (blue) with toluidine blue. However, ꞵ-glucuronidase, α-naphthyl butyrate esterase and peroxidase were not detected in the monocytes, which were negative for periodic acid Schiff reaction, alcian blue, Sudan III, and oil red O. Kenji Kiyono reported the two types of monocytes (as blood histocytes), real and dubious monocytes, from the blood of hagfish Eptatretus burgeri. He observed both monocyte types in grown-up hagfish, but only real monocytes in undeveloped (details not stated) hagfish. His real monocytes in undeveloped hagfish were speculated as the monocytes of lamprey (mistaking the specimen), and the real monocytes in grown-up hagfish as poorly stained neutrophils. The dubious monocytes of Kiyono were considered as the real monocytes of hagfish.

Artur Pappenheim reported a poor description of the morphology of blood neutrophils from lamprey (Lampetra planeri； adult and ammocoetes) and hagfish (Myxine glutinosa) in Folia Haematologica (volume 8, 1909). Here, we inferred on the morphological characteristics of cyclostome neutrophils observed by Pappenheim based on his descriptions and our previous reports. He recognized neutrophilic leukocytes (mature neutrophils) in lamprey (adult and ammocoetes) and hagfish, and neutrophilic myelocytes (immature neutrophils) in ammocoetes and hagfish. This means the existence of specific granules and azure granules. However, the specific granules were considered as an inner layer (L0) of the striated granule (NG2), and the azure granules outer (L1) layer of the striated granule (NG2). The specific granules (=L0 of NG2) would probably have been stained with triacid (purple), acid dyes, and May-Grünwald·Giemsa (MGG； purple), but not with methylgreen-pyronine (MP). The azure granules (=L1 of NG1a) would also have stained purple with MGG, but negative with triacid, acidic dyes and MP.

Monocytes were observed in the blood of inshore hagfish Eptatretus burgei. The monocytes were round or oval, high nucleus/cytoplasm ratio, and have several round or oval eosinophilic granules, which show dark red or blackish red color when stained with May-Grünwald (MG), Giemsa, and MG-Giemsa stain. The granules were positive for alkaline phosphatase, acid phosphatase, and α-naphthyl butyrate esterase. Also, the granules show metachromatic (reddish purple) with toluidine blue. However, ꞵ-glucuronidase, α-naphthyl acetate esterase, naphthol AS-D chloroacetate esterase and peroxidase were not detected in the monocytes, and negative for periodic acid Schiff reaction, alcian blue, Sudan black B, Sudan III, and oil red O. The monocytes engulfed many yeast particles (zymosan).

Two types of stratified (two-layered) granules (type 1, NG1a; type 2, NG2) in the neutrophils of inshore hagfish Eptatretus burgei showed eosin-positive (positive site: outer layer (L1) of NG1a and inner layer (L0) of NG2). In contrast, both eosin-positive sites have been reported to be basophilic. From present result, it is clarified that both sites exhibit as follows: L1 of NG1a, eosinophilic and mataazurophilic; L0 of NG2, eosinophilic, orthomethylenophilic and mataazurophilic. The inshore hagfish neutrophils phagocytosed zymosan particles, in vitro. All blood cells showing phagocytosis were identified as neutrophil.